I think the Gran a PP mentioned was right, this is 'borrowing trouble'.
If I had a girls weekend, my only holiday, I'd planned, paid for and was excited about booked for next year, and my partner said they had a routine operation and if it falls on that weekend, would I cancel it to stay by their side, I'd be pissed off!!
Firstly it's very unlikely, to fall that specific friday before that one weekend! Why spoil my excitement about a rare holiday for me? It's so far ahead, you haven't got date yet, why ask now? Is it a test? You won't get a real reaction to a hypothetical, as i'd be thinking 'it's unlikely' and what else will partner demand about other hypotheticals..?
It all seems very controlling. If it was an operation you were worried about and it definitely fell on same date, then yes, when we talked about how you felt about op, I'd probably cancel my holiday but I would want to make that decision myself at the relevant time.
It is possible your partner was a bit taken aback by your hypothetical demand and joking back with you, given how unlikely it is to happen. Why not wait and see when date comes through?
All you've done is create a resentment for yourself, a suspicion. Really, unless you had other reasons to doubt your partner's caringness , I think you've done both of yourselves a disservice.