diff --git a/specs/altair/sync-protocol.md b/specs/altair/sync-protocol.md
index 8751ba6e8..1aa9316d3 100644
--- a/specs/altair/sync-protocol.md
+++ b/specs/altair/sync-protocol.md
@@ -52,7 +52,7 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
 
 | Name | Value | Unit | Duration |
 | - | - | - | - |
-| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` | validators |
+| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` | validators | |
 | `UPDATE_TIMEOUT` | `SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | slots | ~27.3 hours |
 
 ## Containers
@@ -73,6 +73,8 @@ class LightClientUpdate(Container):
     sync_aggregate: SyncAggregate
     # Fork version for the aggregate signature
     fork_version: Version
+    # Slot at which the aggregate signature was created (untrusted)
+    signature_slot: Slot
 ```
 
 ### `LightClientStore`
@@ -162,15 +164,16 @@ def validate_light_client_update(store: LightClientStore,
                                  genesis_validators_root: Root) -> None:
     # Verify update slot is larger than slot of current best finalized header
     active_header = get_active_header(update)
-    assert current_slot >= active_header.slot > store.finalized_header.slot
+    assert current_slot >= update.signature_slot > active_header.slot > store.finalized_header.slot
 
     # Verify update does not skip a sync committee period
     finalized_period = compute_sync_committee_period(compute_epoch_at_slot(store.finalized_header.slot))
     update_period = compute_sync_committee_period(compute_epoch_at_slot(active_header.slot))
-    assert update_period in (finalized_period, finalized_period + 1)
+    signature_period = compute_sync_committee_period(compute_epoch_at_slot(update.signature_slot))
+    assert signature_period in (finalized_period, finalized_period + 1)
 
     # Verify that the `finalized_header`, if present, actually is the finalized header saved in the
-    # state of the `attested header`
+    # state of the `attested_header`
     if not is_finality_update(update):
         assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
     else:
@@ -184,10 +187,8 @@ def validate_light_client_update(store: LightClientStore,
 
     # Verify update next sync committee if the update period incremented
     if update_period == finalized_period:
-        sync_committee = store.current_sync_committee
         assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
     else:
-        sync_committee = store.next_sync_committee
         assert is_valid_merkle_branch(
             leaf=hash_tree_root(update.next_sync_committee),
             branch=update.next_sync_committee_branch,
@@ -202,6 +203,10 @@ def validate_light_client_update(store: LightClientStore,
     assert sum(sync_aggregate.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS
 
     # Verify sync committee aggregate signature
+    if signature_period == finalized_period:
+        sync_committee = store.current_sync_committee
+    else:
+        sync_committee = store.next_sync_committee
     participant_pubkeys = [
         pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys)
         if bit
diff --git a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
index 04553b3f8..6411ecb6a 100644
--- a/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
+++ b/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
@@ -39,8 +39,9 @@ def test_process_light_client_update_not_timeout(spec, state):
         state_root=signed_block.message.state_root,
         body_root=signed_block.message.body.hash_tree_root(),
     )
-    # Sync committee signing the header
-    sync_aggregate = get_sync_aggregate(spec, state, block_header, block_root=None)
+
+    # Sync committee signing the block_header
+    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
     next_sync_committee_branch = [spec.Bytes32() for _ in range(spec.floorlog2(spec.NEXT_SYNC_COMMITTEE_INDEX))]
 
     # Ensure that finality checkpoint is genesis
@@ -56,12 +57,13 @@ def test_process_light_client_update_not_timeout(spec, state):
         finalized_header=finality_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=state.fork.current_version,
+        fork_version=fork_version,
+        signature_slot=signature_slot,
     )
 
     pre_store = deepcopy(store)
 
-    spec.process_light_client_update(store, update, state.slot, state.genesis_validators_root)
+    spec.process_light_client_update(store, update, signature_slot, state.genesis_validators_root)
 
     assert store.current_max_active_participants > 0
     assert store.optimistic_header == update.attested_header
@@ -69,6 +71,57 @@ def test_process_light_client_update_not_timeout(spec, state):
     assert store.best_valid_update == update
 
 
+@with_altair_and_later
+@spec_state_test
+@with_presets([MINIMAL], reason="too slow")
+def test_process_light_client_update_at_period_boundary(spec, state):
+    store = initialize_light_client_store(spec, state)
+
+    # Forward to slot before next sync committee period so that next block is final one in period
+    next_slots(spec, state, spec.UPDATE_TIMEOUT - 2)
+    snapshot_period = spec.compute_sync_committee_period(spec.compute_epoch_at_slot(store.optimistic_header.slot))
+    update_period = spec.compute_sync_committee_period(spec.compute_epoch_at_slot(state.slot))
+    assert snapshot_period == update_period
+
+    block = build_empty_block_for_next_slot(spec, state)
+    signed_block = state_transition_and_sign_block(spec, state, block)
+    block_header = spec.BeaconBlockHeader(
+        slot=signed_block.message.slot,
+        proposer_index=signed_block.message.proposer_index,
+        parent_root=signed_block.message.parent_root,
+        state_root=signed_block.message.state_root,
+        body_root=signed_block.message.body.hash_tree_root(),
+    )
+
+    # Sync committee signing the block_header
+    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
+    next_sync_committee_branch = [spec.Bytes32() for _ in range(spec.floorlog2(spec.NEXT_SYNC_COMMITTEE_INDEX))]
+
+    # Finality is unchanged
+    finality_header = spec.BeaconBlockHeader()
+    finality_branch = [spec.Bytes32() for _ in range(spec.floorlog2(spec.FINALIZED_ROOT_INDEX))]
+
+    update = spec.LightClientUpdate(
+        attested_header=block_header,
+        next_sync_committee=state.next_sync_committee,
+        next_sync_committee_branch=next_sync_committee_branch,
+        finalized_header=finality_header,
+        finality_branch=finality_branch,
+        sync_aggregate=sync_aggregate,
+        fork_version=fork_version,
+        signature_slot=signature_slot,
+    )
+
+    pre_store = deepcopy(store)
+
+    spec.process_light_client_update(store, update, signature_slot, state.genesis_validators_root)
+
+    assert store.current_max_active_participants > 0
+    assert store.optimistic_header == update.attested_header
+    assert store.best_valid_update == update
+    assert store.finalized_header == pre_store.finalized_header
+
+
 @with_altair_and_later
 @spec_state_test
 @with_presets([MINIMAL], reason="too slow")
@@ -91,9 +144,8 @@ def test_process_light_client_update_timeout(spec, state):
         body_root=signed_block.message.body.hash_tree_root(),
     )
 
-    # Sync committee signing the finalized_block_header
-    sync_aggregate = get_sync_aggregate(
-        spec, state, block_header, block_root=spec.Root(block_header.hash_tree_root()))
+    # Sync committee signing the block_header
+    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
 
     # Sync committee is updated
     next_sync_committee_branch = build_proof(state.get_backing(), spec.NEXT_SYNC_COMMITTEE_INDEX)
@@ -108,12 +160,13 @@ def test_process_light_client_update_timeout(spec, state):
         finalized_header=finality_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=state.fork.current_version,
+        fork_version=fork_version,
+        signature_slot=signature_slot,
     )
 
     pre_store = deepcopy(store)
 
-    spec.process_light_client_update(store, update, state.slot, state.genesis_validators_root)
+    spec.process_light_client_update(store, update, signature_slot, state.genesis_validators_root)
 
     assert store.current_max_active_participants > 0
     assert store.optimistic_header == update.attested_header
@@ -157,9 +210,8 @@ def test_process_light_client_update_finality_updated(spec, state):
         body_root=block.body.hash_tree_root(),
     )
 
-    # Sync committee signing the finalized_block_header
-    sync_aggregate = get_sync_aggregate(
-        spec, state, block_header, block_root=spec.Root(block_header.hash_tree_root()))
+    # Sync committee signing the block_header
+    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
 
     update = spec.LightClientUpdate(
         attested_header=block_header,
@@ -168,10 +220,11 @@ def test_process_light_client_update_finality_updated(spec, state):
         finalized_header=finalized_block_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=state.fork.current_version,
+        fork_version=fork_version,
+        signature_slot=signature_slot,
     )
 
-    spec.process_light_client_update(store, update, state.slot, state.genesis_validators_root)
+    spec.process_light_client_update(store, update, signature_slot, state.genesis_validators_root)
 
     assert store.current_max_active_participants > 0
     assert store.optimistic_header == update.attested_header
diff --git a/tests/core/pyspec/eth2spec/test/helpers/light_client.py b/tests/core/pyspec/eth2spec/test/helpers/light_client.py
index 15c764fc4..b35c6c687 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/light_client.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/light_client.py
@@ -1,5 +1,9 @@
+from eth2spec.test.helpers.state import (
+    transition_to,
+)
 from eth2spec.test.helpers.sync_committee import (
     compute_aggregate_sync_committee_signature,
+    compute_committee_indices,
 )
 
 
@@ -15,21 +19,31 @@ def initialize_light_client_store(spec, state):
     )
 
 
-def get_sync_aggregate(spec, state, block_header, block_root=None, signature_slot=None):
+def get_sync_aggregate(spec, state, block_header, signature_slot=None):
+    # By default, the sync committee signs the previous slot
     if signature_slot is None:
-        signature_slot = block_header.slot
+        signature_slot = block_header.slot + 1
 
-    all_pubkeys = [v.pubkey for v in state.validators]
-    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
-    sync_committee_bits = [True] * len(committee)
+    # Ensure correct sync committee and fork version are selected
+    signature_state = state.copy()
+    transition_to(spec, signature_state, signature_slot)
+
+    # Fetch sync committee
+    committee_indices = compute_committee_indices(spec, signature_state)
+    committee_size = len(committee_indices)
+
+    # Compute sync aggregate
+    sync_committee_bits = [True] * committee_size
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
-        state,
-        block_header.slot,
-        committee,
-        block_root=block_root,
+        signature_state,
+        signature_slot,
+        committee_indices,
+        block_root=spec.Root(block_header.hash_tree_root()),
     )
-    return spec.SyncAggregate(
+    sync_aggregate = spec.SyncAggregate(
         sync_committee_bits=sync_committee_bits,
         sync_committee_signature=sync_committee_signature,
     )
+    fork_version = signature_state.fork.current_version
+    return sync_aggregate, fork_version, signature_slot