Your payment will always be do on the 15th of the month before each package ships. If you know that you will be out of town for the first couple of weeks of a month and won't be checking email, please let us know in advance. If we don't receive your payment we will be very worried about you (plus you won't get any yarn).
ETA: the first shipment will go out at the beginning of February (and after that the beginning of April, June, August, October, and December). We'll post here or on the Ravelry group when we send them out so you can be on the lookout!