Maybe you’ve worked with data warehouses before, in which case the concept of a “Date Dimension” is going to be familiar. If not, the general idea behind a Calendar or Date table is that you have a table of Dates and metadata about those dates. This can include business-specific flags, alternate Quarter structures, alternate Week Start data, or whatever fits your needs
By pre-populating all of the data about a date in a table, it makes querying for specific date-based criteria a lot easier, especially when your fiscal year isn’t aligned with the calendar year or you need to deal with multiple fiscal years. It also helps with those holidays which are aligned with lunar calendars and thus “change date” every year.