A bed-day is a day when a person spends an entire day laying in bed, not because he is sick or because he stayed up all night, but simply because he doesn't feel like getting up. Usually, bed-days are spent watching TV, eating junk food, playing Xbox, farting and downloading new songs on iTunes.
People most frequently have bed-days:
A. when the weather is cold, rainy, snowy or unbearably hot,
B. when they're breaking in their new Egyptian-cotton bedsheets, or
C. when there's a "Law & Order" marathon on.
"Hey! Why weren't you at work yesterday?"
"I was laying in bed until 5 PM"
"What, were you sick?"
"Nope. Just day-dreaming about being in a coma."
A day devoted to nothing but having lots of sex in bed for the whole day.
This could be due to cold weather, so to warm up, sheer hornyness or lack of sexual activity form partner being away.
"Hay lets have a bed day today ;)"