This is a Disco Day taken from your place of work when everyone actually knows your not sick but for some reason just accepts that you're not coming in and does nothing about it.
The evidence of this usually stems from a physical injury sustained over the weekend which allows continued movement and general celebration throughout the weekend but prevents journey to and from your place of work.
Tom did a half marathon on Sunday and tweaked a foot muscle. He made it out on the piss on Sunday evening but hasn't been seen at work since. He must be in a great deal of pain to take a Tom's Disco Day.
A curious phenomenon, most commonly exhibited by supporters of Liverpool Football Club, where an individual is completely unable to see the wood from the trees as far as making sensible, unbiased judgements about their football team’s performances is concerned.
The whole world could see that Liverpool were vastly inadequate but the supporters felt robbed and went to bed dreaming of past European victories and 1990s league wins.