To unlock it as the name says, you have to have eight archers on the 1st day but, it does not need to be on the first island.
I unlocked it on the 3rd day, when I came with the troops from the 2nd island. Keep in mind you will have two peasants ready to put coins to became archers, and that should be almost enough to cover the eight you need. If not just recruit some more as quickly as possible.