I haven't had many Event Day PCNs recently, I used to get them every week (during the season) and this is because residents got to know, so now mostly visitors get caught and they aren't even going to the match.
I would expect the council to fail to prove something at the Appeal stage so forget about the discount if you want to fight as the council are most unlikely to cancel & I win half of my cases by going to Appeal (I win 85% overall, I have fought 2,000+ pcn in the last 7 years).
I would select the ground 'There was no contravention' (or similar wording) and then write that 'signs within the road did not notify me that the day in question was an Event Day, whatever that is, nor where that information was to be found. Please cancel the PCN.'
That is all. Don't veer off into other areas, keep it short and to the point, you only put yourself in it if you write more than that.
If you were loading/unloading or boarding/alighting a passneger who needed assistance that could be an exmeption so do come back if that was actually the case.