3 year anniversary

So it's my partners and I three year anniversary today. We both knew he might have work so we waited until Friday when he got his new roster for work. He didn't have work Saturday so we decided he could have that day to himself since he worked all week. He had work today (Sunday 16th) so we were like cool we can just do something after work then.

Now yesterday Saturday, late at night, his mum informs him they have a family dinner to do go.

It's not a special occasion, not a birthday. But he has to go.

Now his mum knows it's our anniversary because she brought it up saying our anniversary is the day she found out she had cancer or something to do with her cancer. (EDIT SHE DOESNT HAVE CANCER ANYMORE, when I came into my partners life she didn't have it anymore) So she can't really ever forget that date.

Now I understand he still lives at home and everything that goes along with that. But he's an adult. Our of school, works full time, pays rent. Pays for all the stuff he needs, his car etc.

But now I can't see him at all because his whole day was taken from work and now his whole night is taken from this family dinner.

Am I right to be mad? Like I know she knew about it.