Is a family member's birthday a good excuse to miss work?

Basically I am scheduled to work until 10pm this Friday, however I have a huge party I've been planning to attend for a long time. If I said it was my grandma's birthday or something would that be a legitimate enough excuse? I work at a part time job and I'm just starting out. No lectures please I just want answers. Thanks!
Is a family member's birthday a good excuse to miss work?
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