Happy Gazday is a day when nothing EVER goes right and the "Happy" part is used in irony.
You all know the kinda day I'm talking about ? The sort of day that starts off when you get out of bed, late, hungover and half sleeping then stagger barefoot to the toilet only to stand squarely in a pile of steaming dog shit that squelches up between your toes making you want to puke.
Later, after cleaning the shit from between your toes and almost giving yourself a hernia puking, you go downstairs to make breakfast and, if there happens to be a milk carton in the fridge, you end up pouring a lump of stinking semi fermented cheese into your freshly made tea, coffee or cereal resulting in another gut ripping puking session !
The day continues when you go through a succession of miserable experiences and failures at work, school, college, etc. incorporating burns, scalds, bumos and paper cuts and in an attempt to cheer yourself you decide to go to the bar to have a drink and are met by a smiling barmaid/barman holding your massive bill from the night before.
It can also be sung to the tune of "Happy Birthday" when mocking someone who is having a particularly bad day.