A new game came out and he wants to play it. It's the day after the Super Bowl and he is really hungover and queasy from mixing tequila, beer and various fried cheese products. He's exhausted and the only cure is getting through the extras on his Star Wars Blu-Ray box set.
Basically "mental health day" is code for "I need a freaking break to veg out and do nothing for a day." Give him a day to relax and recharge his batteries. Don't take it personally -- we all need a mental health day once and a while to, well, keep us sane.
What do you do on your "mental health days"? Don't worry -- we won't tell your boss.
Play computer games and read all night, sleep all day.
My mental-health days are always nights because the the previous mental-health day has jacked up my sleeping schedule.
SWTOR, read Wheel of Time (since finishing up "A Dance of Dragons"), practice fencing techniques, watch stupid TV-judge shows... oh and nap.
I love mental-health days. I read a lot, sit on the back porch, garden ,(depending on the time of year), watch movies from start to finish, don't talk to anyone ALL day...
I think everybody just needs the feeling of playing hooky once in a while. Something special about taking a non-sick day during the workweek...feels better than the weekend!