Girlfriend (24f) annoyed because she said I’m (27m) cancelling plans we made

My girlfriend and I have been together for 2 years. At the beginning of the month my gf and I spoke about what we wanted to do next month and we made plans for a weekend away at the end of the month, a day out the weekend before that and then for the first and second weekends of the month we mentioned places we’d like to go and things to do but we never mentioned what day we were going to do them. Once we initially discussed the plans, they were never mentioned again so nothing was actually finalised for the first 2 weekends.

I haven’t seen my best friend in nearly a year and we were talking about meeting up and I suggested next Saturday, since I knew I was busy towards the end of the month, and he agreed. I mentioned this to my gf and she got annoyed and said we already have plans. She mentioned the place we said we’d visit and I just pointed out we never actually said we’d do it on Saturday and we could just as easy do it on Sunday.

She said no since Saturday is more convenient so I tried explaining why meeting up with my friend was important to me and she just said I shouldn’t have made plans when we already had plans. I disagreed since the plans were never properly finalised anyway and we’d only mentioned once something we’d like to do.

I said it was still possible to do both of the plans by just doing ours on Sunday but she said no and that I shouldn’t be cancelling plans I made with her but I don’t see it as cancelling since they were never finalised anyway and I’m still saying we do the plans that weekend. How would you handle this situation?

tl;dr I made plans to catch up with my best friend and when I told my gf about it she said we already have plans that weekend. She wouldn’t listen when I said we’d never actually finalised our plans and we could easily do them the following day. How would you handle this?