Grant award Dates (US) I've been in a conversation with a professor about hiring me as a potential postdoc. Recently he sent me an e-mail saying he would like to accept me to his team, but he can do this only if he can get a new grant.

As I understand the situation, his current grant is ending soon, and he hasn't been able to get new grants yet, but is still trying. 

In general, when do US-based foundations announce their grant awards?  (I'd like to know so that I can ask him again on time whether he got a grant, so I don't lose the position that I kind of have right now.)
There's no fixed timeline. For example, the National Science Foundation has deadlines for proposal submission that are scattered around the year, and once a proposal has been submitted, it may take the NSF 3-9 months to decide. There are also numerous other funding agencies.

In other words, the only way for you to find out when that date may be is to communicate with the professor. He may have an idea when that may be, but the NSF doesn't set a fixed date when they will announce their grants -- that email may come on a random day and a random time.