I imagine it’s a luxury from the days before our current standards of climate control.

Imagine you don’t have a central heating unit–you’ve got fireplaces, and that’s it. You wake up, and it’s a brisk 8°C in your house. Staying in your nice cozy bed might not sound so bad.

It may also be a leftover thing from days when people couldn’t afford to stay in bed in the morning. Only the rich could afford to lounge around that way, so having that once a year would have been a way to treat someone like royalty.