It’s so unclear what the % chance of rain means that More or Less once write to the Met Office to ask them, and the answer they gave just confused things even further.
My interpretation of the nonsense they came out with is that it isn’t even really “80% chance of rain”. It’s more that “when weather conditions have resembled these in the past, 80% of those times [over some time period that remains completely opaque] resulted in some rain being experienced”. And the time period is critical to that calculation, but the Met Office’s answer on that score was as clear as mud.
Basically, it’s almost certainly going to rain on Sunday, but it isn’t at all clear that you should expect rain during any given hour of that day.