Hey! It's your character's birthday! Happy Birthday, MC!
Everyone deals with their birthdays differently. I love birthdays. I love presents and decorations. I love balloons and cake. Of course, I never met a celebration I didn't like.
I have known people who despise their birthdays. I once got in a fight with someone because I brought them a gift for their birthday. I didn't know! Some people feel melancholy as they get older, some people feel apathy. Some people forget.
Whatever the story you're working on now, figure out how the main character deals with the day of their birth. Are they happy? Resentful? Sad? Drunk? Risk-taking? Regretful? If you're not in the middle of a story, then write about someone's birthday, and have them do something that reflects how they feel. Pair an action with an emotion.