Calendars
What is it?
A calendar is a set of available trading dates for a certain method of trading. For example, one calendar may include all days every day 24/7 (crypto). Another may contain only weekdays from 9:00AM to 4:00PM excluding holidays (stocks). They allow for the database to retrieve any necessary data that isn't already in the storage, effectively limiting the amount of historical API calls made.
NYSE Calendar
The NYSE calendar includes all weekdays from 9:00AM to 4:00PM, excluding holidays. This is commonly used for trading stocks.