Does vowing to do something for someone who is holding a birthday party count as a vow of righteousness or a vow of contention, what is the ruling on fulfilling it, and does asking about its ruling necessitate anything?

If the questioner made a vow to hold a birthday party, believing it to be a vow of piety, she is not obligated to hold it; rather, it is impermissible, because celebrating birthdays is not permissible. A vow to commit a sin is not allowed to be fulfilled, and there is no expiation for breaking an oath for it, according to the majority of jurists.