Used to say to your homies that
1. it's time for an event to go down
2. it's time to start something that was planned
3. it's time to leave for somewhere that was planned
Origin: Might have originated from the saying 'Make hay when the sun shines'
Ron: I started up the Xbox. You do the first song. Did you get the beers?
Jake: Yeah man! It's hay time!
you say this to greet a friend, especially one you have many inside jokes with. also, this is used to seem showy and cool that you have a friend you can pronounce as loudly as most people say this