It's a whole months work. The scheduling department publish all the different work patterns for the month roughly 6 weeks before and you bid for the ones that you like the look of/ think you can get depending on your seniority . All bids are processed in seniority order and after a second stage of bidding to make your line up to the required hours (called cap) for the month (defined by the company depending work available and pilots available) you have your roster. And not a stupid question . I had no idea what It was before I joined either !