If you purchase directly from Disney then your first year's dues are pro-rated from the later of: the purchase date, the start of your initial Use Year or the occupancy date of the Unit you purchased. You won't be billed until you close on your purchase.
Dues are pro-rated based on the number of days, not months. For example, if on Sep 1st, you purchased a Unit that was already open and you got current UY points with your purchase, you would owe 122 days worth of dues. If you pay dues monthly, your billing might not start until Nov 1st so Disney would deduct the amount in 2 installments.
After the first year, your dues will be due on Jan 15th if you pay in full.