誕生日 and 記念: birthdays and commemorating an occasion
Distinguish a birthday date from commemorative actions and the timing of a celebration.
A birthday date
誕生日, read たんじょうび, means birthday. 誕生日は五月十日です。 gives a birthday of May tenth.
誕生日を友人と過ごしました。 means spending a birthday with a friend or friends. It does not necessarily mean holding a large event.
Marking something to remember
記念, read きねん, means commemoration or keeping an occasion in memory. 卒業を記念して写真を撮りました。 means taking a photograph to commemorate graduation.
旅の記念に切符を残しました。 means keeping a ticket as a memento of a trip. It does not state a plan to use the ticket again.
Date and celebration time
誕生日は明日ですが、今日お祝いします。 says that the birthday is tomorrow but the celebration is today. The dates can differ.
記念に何か買いたいです。 expresses wanting to buy something as a memento. It does not identify an item or establish a completed purchase.
When is the birthday in 誕生日は明日ですが、今日お祝いします?
Show the answer and why
Tomorrow. The celebration is today, but the birthday itself is tomorrow.