I would think that waiting until January to purchase tickets for May should be no problem. They should still have lots of availability that many months ahead.
In the meantime, be sure to sign up for their emails, like their Facebook page, etc. to make certain that you know of any promotions they might be offering. That way, if a really good deal comes up between now and January that applies to your travel date and you want to jump on it, you can do so.