This discussion has been made a lot of times earlier.

DB's forgot Will's birthday as S2 was made 5 yrs ago for them also as it was mentioned only once, not that important to plotwise.

Solution- They told, if they get permission, they will change the S2 line to May 22nd.

• Sobs in stranger things *

There was also discussion that it's possible that simply forgetting his birthday could have added to his depression that day, and explain why he was so upset with his friends. Makes sense, as I would be pretty miffed if my friends forgot my birthday.