The sign for the bay you parked in is in two parts, one part tells you no waiting or loading during events, then there is a second part, added on at the bottom, telling you the dates and times. This second part should be added some days before an event is to take place so that anybody parking beforehand, and intending to leave their car will know whether they can leave it or not.. So when you parked up the previous day, did you go and look at the sign ? If you didn't, you have failed in your obligation as a motorist parking on a yellow line to seek out the restrictions sign for the bay to make sure you could park. If the council are only putting the event day date(s) and times at the time they start thus giving no notice at all, then they are acting unlawfully. Do you have any proof of no event dates being placed on that sign when you parked ?
Interestingly, there is a zone for Hampden events, and signs at the entry as per here: -
https://maps.app.goo.gl/qBJB8nHRk2XHSPpR6The event dates are on these signs, and when these are updated, the sign where you parked, and also the other bay opposite, should be updated at the same time.
Secondly, after service of the PCN to the car, the next legal obligation on the council is to send a Notice to Owner after 28 days have elapsed with them receiving representation, or the PCN being paid. There is no obligation to send anything at 14 days.
The sign does not just indicate No Loading ,because it also displays the symbol for No Waiting alongside the no loading text.
So the obvious question to ask is, have you submitted any representations against these PCNs, or have you just coughed-up the discounted penalty ?