Samsung
Add Birthday to a Contact auf Samsung
Samsung Contacts does not show a birthday field on the standard edit screen. The option sits under a collapsed section, which is why many users assume the feature does not exist on their phone.
To reveal it, open Contacts ➔ Choose a Contact ➔ Tap on Edit. On the edit screen, tap View more to expand the hidden fields, then select Important dates.
Inside Important dates, tap Date (Birthday) and enter the day, month and year in the picker. Confirm with Done, then tap Save on the contact edit screen to store the change.
Skipping the final Save tap is the most common mistake - the date picker closing does not save the entry by itself. If the contact is linked to another account, such as Google, the birthday you add locally may not sync back to that account automatically.
This method works for any contact stored on the device, regardless of whether Samsung's built-in Calendar app is used for reminders.