Cat due to be spayed tomorrow but still in heat

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Hi all, my cat is 8 but un-spayed. She's an indoor cat and never had a litter (please don't judge, I'm trying to do the right thing by her now.)
This has not caused us any issues apart from infrequently going into season for a few days. Recently this has got more frequent and I can see it's causing her stress so have opted to have her spayed.
When I booked, the receptionist said "ideally" not in season. I thought she would be done by now but she isn't. Should I postpone the procedure?
Thanks for any advice.

Our vets refused and said they won’t do it when in season. You would be better to phone them tonight to check. It can be “slightly more risky because blood vessels around their womb tend to be a bit bigger” so they may ask you to rebook. Phone them and see.

Thanks, that's helpful. I'll give them a ring when they open at 8am. Hope they can re-schedule for soon.