As a writer of middle grade fiction, do you need to think about your character's birthday? Definitely. Birthdays are hugely important to kids. This is especially true between the ages of 8 and 12, when kids start to think more about planning parties and who might be invited. Kids in this age group may have themed parties, e.g. animals, Harry Potter, or go on a special outing, e.g. bowling, cooking class, laser tag.
Even if you aren't including a birthday party in your story, thinking about when and how your character celebrates their birthday can help you to develop their personality. Some ways birthdays may be significant:
1. A important life event may occur when your character becomes a specific age. Think about about family traditions -- when you got your first watch or were allowed to stay up later. Or how characters may develop special talents or powers at a given age. Characters may dream or talk about what they will be able to do, once they get to that next age.
2. Birthdays are a rich source of conflict or power. Kids may say "I’m not going to invite you to my birthday” to be hurtful or when they are feeling hurt. Or they might tell a friend "I'm not going to come if you have a _______ party" as a way to show power.
3. Birthdays reveal something about a character's relationship with their parents. For example, a kid may ask their parents to invite kids they barely know, just to get more presents. Or they might bargain for not having a party if they can have a bigger gift.
4. Birthday gifts can help reveal attitudes and emotions. What does your character want for their birthday? Something big and expensive or small and meaningful? How do they act when they receive something different than what they wanted?
There are many books where birthdays are significant or the beginning of something new in a character's life. A few examples:
Harry Potter and the Philosopher's Stone by J.K. Rowling
Faires and the Quest for Neverland by Gail Carson Levine
The Indian and the Cupboard by Lynne Reid Banks
11 Birthdays by Wendy Mass
Savvy by Ingrid Law
If you want to include some birthday party details in your story, this site might give you some ideas:
Has thinking about a character's birthday helped your writing?