An Off-day is for sure an off-day in Cargolux.
But, you might (very rarely) come home a day or two late from a trip. Almost never happens. If it happens it depends which off-days you come home on. On your 6-day block you have to agree to come home late (or they have to commercial you home), and if so you will get 2 for 1 day. On your 3-day block they don't need your agreement and the days are compensated 1 for 1.
If they are really short of pilots (due to sickness or delays) they sometimes call people on off-days or vacation to ask if they are willing to fly. Again, 2 for 1. Some people do some don't. That's why they always call the same people...
You might end up having blank-days (not off and no duty). Here they can call you out, but latest the day before as far as I understand.
But to make it short, on your off-days you are off, like in any other airline. Did 738-Midranger understand it defferently?