The ABSOLUTE earliest time for you to look would be MID JUNE.
Leases usually start the 1st of the month, regardless of when the person moved in.
Most leases have "Tenant must provide 45 days of notice if they decide to vacate"
Thus, by Mid-late June, most of the August inventory has revealed itself.
You must know, most of the country has about a 10% vacancy. Manhattan has LESS THAN 1% VACANCY. SUMMER APARTMENT RENTALS IS THE ONLY TIME YOU WILL PROBABLY EVER DEAL WITH SALESPEOPLE (BROKERS) WHO ARENT KIDDING WHEN THEY SAY YOU CAN'T WAIT OVERNIGHT TO THINK ABOUT IT.
The apartments for August will thenl rent in the following order:
Best apartments that are nice (People who grab quick)--------> Worst Apartments that meet people's minimum space requirements (People who "don't prepare", "don't get with a good, reputable broker", "Don't realize what position they are in to negotiate", "people who think they will find a great no fee deal in Manhattan", until it is too late.)
I've been playing this rental game for a while now, I'll be happy to help you win! you can reach me at 803-309-3470 to discuss further