I went to donate at the beginning of March as soon as it had been more than six months from my return from Thailand. (That's how long you have to wait.) Unfortunately my iron count was too low! Got to wait three months now so I guess I'll try again in June!
There's a session on 25/1 (this Sunday) that I could go to. I asked mum and she said to have a really good diet before and after so I don't crash or anything. I might try doing the diet thing for a while and then go next month? I don't know when the best time to go would be, cycle-wise, or if it would even make any difference.