Its probably relevant to say that I'm a 21 year old girl and my boss is a ~40 year old male. I have been working on a retail job for almost 2 years. Its a small shop and apart from my boss only I and another man work there. On my first year of employment, I was told that the two of them usually go out for dinner for a Christmas celebration every year in December, but last year it didn't happen because of the pandemic... this year I couldn't avoid it and was asked what day I had available for the dinner. The problem is, this month my colleague was let go and I'm the only one working there right now. I'm very introverted and have trouble understanding social situations like this, and even with my ex-colleague I was uncomfortable, but now the idea of going out for dinner with my boss just sounds odd. I wasn't given the opportunity to deny it because I wasn't asked if I would like to go, he just asked what day I would have time for it. I don't know if this is a weird or unprofessional situation but even if it isn't I don't even know what we would talk about because of our age and life stage differences.

TDLR: How can I politely decline a free dinner with my boss in a way he doesn't insist on it?