Christmas day spent in bed with your lover playing with his package; spending Christmas day in bed with your lover and forgetting to open presents until the day after; if you're a guy then it's getting a taste of 'the' cookie on Christmas day; celebrating Christmas in a nontraditional manner where swapping presents is substituted or augmented with so much kissing, foreplay and sex that the couple forgets the customarily accepted gift exchange that usually takes place on December 25th ... or is that the 26th? Well any way, Christmas day becomes Kissmas day and the Christmas gift exchange takes place the following day.
Dave whispers, "Merry Kissmas, Sheila." as he pulls her close to him in bed and kisses her softly thus starting a Christmas sex session followed by spending the rest of Christmas playing around in bed and drifting in and out of sleep. Dave is too busy getting a piece of Sheila's cookie and Sheila is too busy enjoying Dave's package neither one cares they don't get around to opening Christmas presents until the day after Christmas. Ergo Dave and Sheila celebrated kissmas not Christmas.