The drawing is around the 20th of the month but it will be for postcards received between 20 Nov and 20 Dec. Your post card will be eligible for the next drawing in Feb and if you win you'll receive your prize sometime late March or early April.
Folks who are receiving prizes now had their cards picked in Nov...the rules say prizes sent 55 days after the drawing date. Patience is one of the 3 Ps that folks who enter sweepstakes talk about, Postage, Persistence and Patience.
I have tried to read the official rules re: this and I get out of it as this: each day the mailed in postcards are bundled up and the date is put on them when received. If, on any given day, the mail-in entry pool is randomly selected, they will pull that day and pick out the postcards. Your postcard is only good for that day.