The built-in calendar in Spark works only with the following calendars:
For example, if you want to use your Google calendar, please add to Spark the Gmail account to which this calendar belongs.
For other accounts such as Yahoo, Outlook or custom IMAP, the option to enable calendars in the settings will be greyed out.
To learn more about Spark calendars, please check out these articles:
How to turn on and view calendars in Spark.