Show the Birthdays calendar on iCloud.com
Calendar includes a special Birthdays calendar that can display all birthdays that you have in iCloud Contacts.
In Calendar on iCloud.com, click the Action pop-up menu in the bottom-left corner, then choose Preferences.
In the General pane, select “Show Birthdays calendar,” then click Save.
To show or hide the Birthdays calendar, select or deselect it in the sidebar. You can also see the Birthdays calendar on your iOS device, iPadOS device, and Mac by selecting it in Calendar.
To modify a birth date, make the changes in iCloud Contacts; changes are reflected in iCloud Calendar within a day. See Edit a contact on iCloud.com.