Weekend supplement charge

So this evening got home to find my invoice from VH for our fly drive holiday next year.
We are flying out on a Thursday and returning home 4 weeks later on the Thursday, clearly landing at LHR early on Friday morning.
On my invoice is a weekend supplement charge. Now call me silly but I was of the belief that a weekend supplement charge was only used when flying at the weekend. Therefore surely I shouldn't have been charged for this? Or is it some kind of 'luxury' as I am holiday for a period including a Saturday night stay?
Answers would be appreciated.
We are flying out on a Thursday and returning home 4 weeks later on the Thursday, clearly landing at LHR early on Friday morning.
On my invoice is a weekend supplement charge. Now call me silly but I was of the belief that a weekend supplement charge was only used when flying at the weekend. Therefore surely I shouldn't have been charged for this? Or is it some kind of 'luxury' as I am holiday for a period including a Saturday night stay?
Answers would be appreciated.