It's still snowing — I'm not going to work! I'm going to take a ___ day.

The correct answer is: duvet

If you pretend to be sick so you can stay home from work, we say that you are taking a duvet day. A duvet is a bed cover. 

According to Peter Mooney of the Employment Law Advisory Services, the highest number of duvet days are in February. The first Monday in February is the day when staff are traditionally most likely to pretend to be sick so they can stay home from work. In the UK, it's nicknamed the "National Sickie Day".

If you suspect that a colleague is pretending to be ill, you can also say "she's throwing a sickie".

take a duvet day = einen Tag blaumachen

throw a sickie = blaumachen

When someone really is ill, we can say that someone has called in sick or is taking a sick day. 

call in sick =  sich krankmelden

take a sick day = sich einen Tag krankmelden