I don't think it matters that the school arrange it a year in advance - it's still a saturday and people still have things that they commit to on a regular basis on weekend days - just as they do after school and if the school tried to make you stay late for something.
I would tell them that they didn't give you enough notice, that you have an ongoing commitment on a Saturday morning so won't be there, and that if they are thinking of having it on a Saturday morning next year too, then you're giving notice now that you won't be able to make it then either as you know that your commitment will still be ongoing.
If they do try to fine you (which I thought was only if you took dc out of school for more than 5 days holiday or lots of absences, rather than a single morning/day) then I would ask them to show you the relevant law that enables them to fine you for not sending your child in on a non-compulsory school day.
If they find the law then unfortunately it's tough
but chances are there isn't one and there will be nothing they can do about it. They have absolutely no right to demand that children are in during non-designated school hours.
And if they get snotty with you about it, ask to see their risk assessment for having their sports day in 28+ degrees heat, blazing sun, (no shade? minimal shade?) particularly for young dc (who overheat more quickly than adults and will have more sensitive skin), pointing out that you believe that running an event in such conditions is a serious failure to safeguard the health and safety of pupils, teachers and parents. As such, you have done your own risk assessment and found that as a responsible parent, there is no way that you can allow your dc or yourselves to take part in such an event, whether it is on a Saturday or a Tuesday - it's still a safeguarding failure.
I bet if you put something along those lines on the facebook page you would get a lot of others that would join in and agree with you.
Bummer for the school and their event, but that's what happens when you have a summer sports day. You frequently run the risk that it will be rained off - and occasionally run the risk that it will be too hot to have it safely.