That is not uncommon at all. From all my research a lot of people tend to have theirs on a schedule. Mine were worst at 8-9pm and then first thing in the morning on the same days.
Try, like Nicky mentioned, to keep up with what is going on during those days and right before. A lot of people experience what is referred to as "weekend or letdown" headaches...something like the opposite of "stress" headaches...where once their big stressors are down such as a big work project or now at this time of year a Holiday...they will notice that the headache sets in shortly after things settle down. Mine can do that or just the opposite, when I get extremely panicked from stressful events I can feel my pain flare up-I like to think of it as a side effect from the adrenaline rush that I am getting from the upset of whatever is going on.
Keep notes like that in your Journal-that may very well help you pinpoint. Then again, they are migraines and come when they want to...yours may just be on a 3 days schedule. Drink plenty, keep a tight sleep schedule and TALK TO YOUR DOCTOR about Frova, it can be taken before a migraine to prevent it in situations like that.