In reply to smile youve won:
One thing to watch out for, is easter. They take it pretty seriously over there. I think there's less ferries on those days/weekend.
I agree with the others, just head to Kos the night before and stay at a hotel there if your flight is early.
There's also the issue with rough seas/wind in April. Last time I was there, someone almost missed their flight because a storm came in. The ferries stopped for a bit, but luckily he managed to get one.
So going the evening before gives you a bit of leeway in that regard.