serious question

I work 3rd shift in a factory. My shift is regularly 11pm -7 am. Even though my title is for 3rd shift my crew and I are the first to come in to work after shut down days, then we are followed by 1st shift.

On Mondays if we have not worked the weekend then my crew and I are told to come in 4 hours early and help make sure the machines are in working conditions. Normally this isn't a big problem, the over time is nice. But this up coming weekend for memorial day we are being told to come in 4 hours early for my tuesday, but that would cut into my holiday. And I know we don't get paid for it even though we are still coming in on the day of the holiday.

My question is, is that even legal?

The way payroll is set up is that if anyone on 3rd shift clocks in early then that time applies to the following day.

I don't know if I'm explaining this correctly or not. But if anyone could help shed some light on this I would greatly appreciate it.