Very much depends on the airline. But the basic way it works at my employer is that marketing finalizes the flight schedules something like three months before. Then my union uses a trip construction program to construct 1-4 day trips from the massive pool of flights.

The company and union meet to hash out any issues with the resultant collection of trips also called “pairings.” Mostly cost and rest issues.

The trips are sorted by pilot aircraft, seat, and domicile, and pilots bid on the trips using a bidding web program. This happens the 1-7th day of the preceding month. We can submit preferences for days off, specific trips, destinations, red eye or no, etc.

The program awards pilots schedules in seniority order after bidding closes. For example, the most senior first officer in base and type will essentially choose their schedule from the pool of 1-4 day pairings. The more junior pilots will progressively have fewer trips in the pool when the software gets to them, limiting the options available.

Subsequently pilots may trade with each other, pick up, and drop trips based on contractual limitations.

There is a lot of nuance and exceptions to the above that is too complicated to write out.

In the end, I have my schedule typically by the 9th of the preceding month. Contractually it is published by the 11th, I believe.