If United and/or Liverpool progress their game could be on the Tuesday so wouldn't be able to play on the Sunday, so I'm guessing we would swap to the Sunday, as if we progressed the other semi would then be on the Wednesday.
spotted this one Wiki - Should Manchester City and Manchester United qualify for the semi-finals, the order of legs will be reversed with Manchester United instead hosting the second leg.
As i read that, if United get through they have to play Tuesday/Wednesday 7-8th May @Home so we def cant be Tuesday 7th at Home. Therefore, we are very likely to be at Home 30/1st and Away 7-8th.