In reply to An Exiled Northerner:
Were you riding in dirty shorts?! (no sniggering at the back, I mean ones that have been ridden in once before for example.) Or did you get very sweaty and then go and sit in your shorts to have a coffee for an hour.
Just a couple of ideas for what the cause might be if no other change.
Relief will depend on the sore. - if it is a small spot like sore then a corn plaster may help. If it is a broader red lump then probably not.
At this stage I'd keep your bum well aired (no tight boxers) and clean between now and Sunday to maximise healing and then suck it up for Sunday and reassess.