Each aircraft has it's own set of start days. There are 3 schedule bid periods now that don't change your aircraft or seat so you can switch between 7 and 7 lines and/or the 15 day reserve or 18 day schedules 3 times a year. There are published seam tables for those who change between 7 and 7 lines since it isn't a very smooth change most of the time- especially if you change (for example) from one Thursday start to the next weeks Thursday start- they won't work you two straight weeks or give you two straight weeks off to accomplish the change over, but the contract dictates exactly how they will do it.