A holiday; the one day each year we set aside to get together and give slaps.
Typically coincides with (and is most effective on) Thanksgiving, but can be applied to any day of the year. Thanksgiving makes for the best usage, as it can help put a different spin on a typically insane holiday, and makes revenge slaps all the more ironic.
"Hey, remember that one Slapsgiving when you slapped Dan across the face while he had his mouth full of cranberry sauce?"
"Totally! It was the best Slapsgiving ever!"