By Kristen Kidd Special to The Denver Post April 1 is National Sourdough Day. No fooling. The curators at NationalDayCalendar.com mark occasions to commemorate every day, and Saturday is sourdough’s turn. (Today, you could toast lemon chiffon cake, manatees and mom-and-pop business owners, for instance.) Of course, mom-and-pop business owners Jeff Cleary and Kathy Mullen pay homage to sourdough every day at their artisan bakery, Grateful Bread Company, in Golden. This Saturday, the bread-mongers will step up their ongoing tribute to sourdough with a large variety of fresh baked breads and pastries featuring the special ingredient they’ve been carefully cultivating since Y2K: their all-natural sourdough starter. Seven days a week, they bake for local chefs, but on Saturdays Grateful Bread also opens to the public from 10 a.m.