誕生日

1. Origins, structure, and basic meaning

• Structure: 誕生日 (たんじょうび) is the annually recurring date of a person's birth. 誕生日を祝う makes it the celebrated event; 誕生日に marks time.
• Basic usage: A family celebrates the speaker's birthday with cake; a handmade card is given on a friend's birthday.

2. Common pitfalls and similar expressions

• Anniversary celebrated and date chosen for a gift
  • The first makes the birthday an object of celebration; the second uses に to time the gift. Focus is the recurring date rather than the birth year.