Start a Core Challenge this weekend, and build a lean, toned core throughout the holiday season!  Ready… Set… Go! Day 1:  Hold 20 seconds Day 2:  Hold 20 seconds Day 3:  Hold 30 seconds Day 4:  Hold 30 seconds Day 5:  Hold 40 seconds Day 6:  Rest Day 7:  Hold 45 seconds Day 8:  Hold 45 seconds…