What is a MCAL File Type?
You will probably only encounter a MCAL if you play Monika After Story and browse the directory of the installation of the game. The file can be found in the following directory:
Appdata%\ RenPy\ Monika after Story\.
The MCAL files are not opened manually. Instead, it is referenced by Monika After Story to load the dates into the game.
What is a Default MCAL File Name?
Db.mcal - File name by default for the calendar file containing all special dates referenced by Monika After Story.