...when it's too cold to send the kids outside!
My oldest is 5 and only goes to Kindergarten half day so this wasn't too much of a change for us. But it was still an extra three hours having her home instead of at school. Three extra hours to fight with her 2 year old sister and bug me to go play outside in the snow... except that it was only 9° (with a wind chill making it feel like -5°) so I couldn't send her outside!
If you have older kids, hopefully you have all the goodies- video games, computer, ipod... stuff to keep them entertained. And hopefully you also have plenty of food and snacks on hand to keep their tummy's full- hungry kids are always cranky!
If your kids also have a snow day, what will you be doing?