To make sure we never forget a birthday, we can also add the birthday in the contacts. The field will not be visible until we tap "Show more" in edit mode. Under Important Date, we can not only create the anniversary or a custom type, but also select the birthday.

Note: If you want the birthday to be displayed in the calendar as well, the contact must be saved in the Samsung account.

Android 10 (One UI 2.0)
Step 1:  Open the  Contacts -App
Step 2:  Tap on desired  Contact
Step 3:  Tap on  Edit
Step 4:  Tap on  View more
Step 5:  Tap on  Important date
Step 6:  Tap on  Date
Step 7:  Enable  Enter year
Step 8:  Set date of  Birthday  and confirm with  Done
Step 9:  Tap on  Save
  1. Open the Contacts-App
  2. Tap on desired Contact
  3. Tap on Edit
  4. Tap on View more
  5. Tap on Important date
  6. Tap on Date
  7. Enable Enter year
  8. Set date of Birthday and confirm with Done
  9. Tap on Save

Samsung Instructions