It would suck if I was called in on a weekend (without having agreed to be on call). It would suck even more if I was called in for a whole day just because I'd demanded that to be my minimum. Instead, weekend hours could be priced 16 times higher. There could be a quantity discount so that a full day would be somewhat less expensive.

>It would suck even more if I was called in for a whole day just because I'd demanded that to be my minimum

This isn't really how it works though. If you find "getting called in" (what does this mean?) to be terrible, you should be charging more for that service. You should be in control.

The whole point is to generate enough surplus value that your client signs the cheque, no questions asked, and is happy to do it. You shouldn't squabbling over hours.

I don’t think it means you work a full day, but that you bill a minimum of a full day for any weekend work.

A while back I worked at a consultancy that typically billed using a 15-minute granularity during standard office hours, reasonably rounded up/down. Unless otherwise contracted, outside of office hours the granularity increased to 1 hour rounded up. A 5 minute fix would be billed as an hour, a 65 minute fix as 2 hours, etc. It was also only undertaken for emergencies; general work requests would be done the next business day.

Generally all contracts were budgeted some number of hours per billing cycle per service. N full-time engineers would be converted to an equivalent amount of hours for the budget.

The result was that overtime was used judiciously and clients were rarely insisted on pushing big changes near the end of a day or on Fridays.