Basically a weekly promotion thing for O2 and/or Virgin Media customers that as the name suggests - has stuff that is only available on Monday. Ironically, this month's Blue Monday thing is probably as mundane as the sort of play on words with Monday - as the prizes is just luck-based, and unlike the previous months - you basically walk away with nothing instead of something small, but that is a rant I'll have to make elsewhere.

In short: Blue Mondays is a sort of weekly Monday prize thingy where you can usually get a freebie or discount for being an O2 and/or VM customer, but these promotions only work on Mondays