There is no mandatory time to come to FINNS, though if you choose an arrive before 1 p.m. slot for your booking, you must arrive before 1 p.m., or you may lose your seat (you won’t lose your deposit, though and can still spend that if you get here late. Deposits are only forfeited if you fail to turn up completely).
We always recommend coming to FINNS as early as possible; that way, you can enjoy the sun and the pool before sunset, when the party really starts to get into full swing.