The way I understood it is if City and United went through, we cannot both play on the same night so one of us would have to play on the following Tuesday, meaning Saturday was the only option.
It can't be 12:30 because that is BT's. Sky's only game on a Saturday is 5:30.
I think the police would prefer it to be early KO on the Sunday which they will move it to if we play Wednesday, but the only way I can see that happening is if they get lucky with the draw or try and do a deal with UEFA (unlikely).