Okay in Chiang Mai, Bangkok and a few other immigration offices you can apply for an extension of stay when you have 45 days or less left on your current stamp.
As has been pointed out there is no down side to going early because they ADD the new extension to your old expiration date.
@Molly ********
, you'd be way better off going to get your extension the week before the Songkran Holiday. I'm sure the week of the holiday is going to be a nightmare out at the immigration office.