Every Month the flying schedule changes and you get to "Bid" on what Pairings you want to fly.
The pairings available are posted on the morning of the 10th for the following month and you go into what's called PBS to bid on the trips you want. the pairings are assigned based on your seniority in base and in seat. DEN CRJ CA seniority 1 gets awarded the trips he/she wants, then PBS assigns DEN DRJ CA seniority 2 his/her choices based on what's left and then down the line. Same thing with the FO's. Each seat and airplane and domicile has it's own seniority order and pairing package.
You'll have from 9:00AM Saint George UT time on the 10th to 9:00AM the 15th to bid for what you want. A soft schedule comes out usually the evening of the 17th, with a firm schedule posted to SKED+ (often referred to as Sked minus) on the 22nd at 8:00PM SGU time, when you can then try to drop, trade, post or pickup to/from "Open Time"
There are also monthly Transfer/transition/ upgrade awards with a Domicile Vacancy Report posted around the 20th, with awards given by the end of the month for moves or classes 30 days or so later. Those awards are based on your seniority, however transfers are usually awarded before upgrades/transitions as classes are not always available, so seniority does get trumped by seat qualification quite often. Break out the popcorn for the responses to this abrogation of seniority.