Hi op,
I remember having a 6 month old baby and my MIL decided to take my husband on holiday for a week.
It nearly resulted in us divorcing to be honest.
I was so utterly tired, stressed and upset.
He worked Monday to Friday and I only really got a break at the weekend. Even just time to hop in the bath for a bit longer, or make proper decent food, knowing our son was safe with his dad. Mon - Fri was tough and I barely saw an adult human in that time. I lived for the weekends.
In my case the holiday was too big. Too invasive on our family. Took my relationship too close to the edge. I blamed my husband for going, but I thought MIL was stupid or selfish for taking him, and whilst eventually my relationship with my husband healed, my relationship with mil did not, I held too much resentment. Twenty years on I don’t bother with her. I see her at Christmas and birthdays. By default (because it always used to be me reminding DH to visit his mum, buy her a gift, send her a message etc) DH hasn’t bothered with her either and I make no effort to remind him to anymore. Last time he saw her was Christmas.
What you did was small scale by comparison, but I agree, taking their only family time and leaving her for yet another day alone with a young baby was probably a massive piss off for her.
I don’t think it’s your job to take her out now though. You don’t have to offer adults rewards. But I think next time offering to babysit, so they can enjoy time together is a much better idea.
Of course you’re also entitled to spend time with your son. I think perhaps just check a suitable time with the pair of them before booking anything, so she has that chance to say ‘I’m knackered, please not this weekend’.
I don’t think it’s a huge deal in the grand scheme. She’d said how she feels and you’ve responded appropriately. I’m sure it will soon be forgotten.