Hi viktoriaholmes you probably saw this post from Jack. Time Period using Days | OneStream Community . If you want to alter the Periods in a Year (M1 - M12) or the days in each month, that is best done when creating the Application so you need to know that up front before your application is created. It is a Custom Time Profile. System tab in OS > Time Dimensions > when you click on that you can adjust which 'Week' falls in to which Period for which Quarter. Jack discusses the challenges on maintenance and application performance in his post, which is worth considering.
Regards