Regarding PS+ Instant Collection...

Hey polynauts!

I have been a reader of this site since it started, but just created my account like 2 minutes ago.

I have just ordered a PS Vita that will be arriving in a few days, and I am planning to get a PS+ subscription as the instant collection is very attractive. One question I have though is that whether I will be able to download games from the instant collection from previous months as I am scared that I will miss the chance to get the games from this month's collection. Must I download the games I want before the next month's collection arrives?

Thanks for any help you guys can provide!