29th of August seems be the correct date. I checked three books, which all gave that date. The books:
- Suomen rannikkotykistö 1918 - 1958 (Finnish Coastal Artillery 1918 - 1958), page 260
- Rannikkotykistö taistelee (Coastal Artillery in Combat), pages 18 - 21
- Koivisto ja Viipurinlahti 1941 - 1944 (Koivisto Island and Viipurinlahti Gulf 1939 - 1944), pages 208 - 209.
As one can guess just from the amount of pages, Rannikkotykistö taistelee contains detailed information about the particular battle. Most important parts:
- Finnish recon patrol had succeeded slipping to the island unnoticed in night of 27 - 28th August. Due to information that it provided, commander of Coastal Battion 2 decided to attack the island immediately and without artillery preparation.
- The Finnish unit attacking to the island was reinforced Rannikkopataljoona 2 (RP2 / Coastal Battalion 2) commanded by Captain V. Vuorela.
- The reinforced RP2 contained:
--- 2 strike companies (iskukomppania), basically rifle company of coastal infantry with special training for this sort of attacks.
--- 1 rifle company
----- 1 Machinegun Company from Linnoituspataljoona 3 (Fortification Battalion 3)
----- 3 x 20-mm Madsen automatic cannons
----- 1 antitank-rifle
----- 2 squads of engineers
----- artillery fire orders team (for calling in artillery support)
Total strength: 425 men
The landing area was in west coast of the island north of Kiuskerinsaari island. First unit to land was to be 2nd Iskukomppania, which had the mission of capturing Kiuskerinsaari and securing a beachhead in Teikarinsaari Island and capture the island in there. 1st Iskukomppania would arrive as 2nd stage, capture northwest part of the island and sent recon to southern part of the island. The 3rd stage of the attack would be landing of the rifle company and most of machinegun company and depended development of the situation. Artillery support was one artillery battery of four 75 K/17 field guns located to Vilaniemi Cape.
The attack started 01:30 29th of August. Kiuskersaari Island was captured with a Soviet patrol found there taken was prisoners of war. The landing to Teikarinsaari Island succeeded also, but apparently the Soviets spotted since they started firing flares from northern part of the island. 2nd Iskukomppania half platoon behind to secure the beachhead and headed to village surrounding it from north and west, the first skirmish happened there and was followed by a Soviet counter-attack. The counter-attack was repulsed, but it halted advance of 2nd Iskukomppania.
1st Iskukomppania landed soon after the 2nd Iskukomppania. Most of 1st Iskukomppania headed to northwest part of the island, while its smaller part headed towards southern part of the island for the recon mission. The company succeeded forcing the Soviet troops to retreat in end of the north-west cape, where the Soviets had a base. Attempted counter-attack was repulsed and once reinforcements started arriving the base was captured by 04:30
Once reinforcements had arrived the village was captured by attacking it from south and southeast. The Soviet troops defending the village retreated towards northeast cape, where the battle continued until 12:30. Small number of Soviet soldiers succeeded escaping from northeast cape of the island by boat.
Around 11:00 the Soviets attempted approaching the island with boats escorted by gunboats. The attempt was repulsed with fire of two machinegun platoons and some captured antitank-guns.
According Finnish intel the island was defended by a reinforced rifle company, which had about 100 men. According estimates only about 20 men had succeeded escaping. During the attack Finnish artillery support had proved too inaccurate, so after brief use it wasn't used anymore. Soviet troops defending the island received fire support from gunboats and artillery battery located to nearby Tuppura Island. Finnish losses for this battle were 8 men killed in action and 16 wounded.