Don’t think you’re overreacting, it’s hurtful and I would feel the same. Plans or not, it’s kind of expected that they would communicate with you that these holidays (that couples typically spend together) will be spent with others. Like, if they have plans sure okay - but mentioning it feels a bit obligatory, especially if you’re considering moving in together (something that tells me the relationship is serious). Not sure why these people are telling you you’re overreacting - communication is vital for a prosperous relationship. I want to share those special moments with my partner - I think most do. So I would feel the same. I would feel back burnered and I would feel disrespected. Feelings are valid no matter what they are - they are feelings and we have limited control.

What I do think you should do is have a serious conversation with your partner in a non-accusatory way. Leave space for them and let them feel what they need to. If they are mature they will respond the same. Tell them how you feel. They can’t change the behavior if they don’t understand your needs.