Should he be mad?

Shea
My SO and I have been dating for 10 months. Our "anniversary" is on the 18th. This month will be our 11th month of dating. Today I told him I was going out of town to see my sister in three days for her birthday with my bestfriend. Should he be mad that I only gave him a 3 days notice, or is he freaking out for no reason. 
Btw, i doubt he had anything planned for our 11th month.. he never has before. If he wanted to start celebrating it, it would be on our one year. AND a week earlier I asked him if he wanted to go to the beach in a few weeks for my sisters bday. 

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