Mac Calendar requires setting Birthdays Calendar every time Mac startsup

iMac, running Catalina 10.15.7.
I have recently realized that Calendar isn’t showing birthdays I have set in Contacts.
I go to Calendar Preferences click on Show Birthdays Calendar and everything is OK till tomorrow, when my Mac startups again and Show Birthdays is off.
Anyone else has the same problem or found a solution?
Thanks for any help.