A steady flow of shoppers strolled through the Harvard Square Holiday Crafts Fair on a recent weekend, eyeing the bounty of cozy knit hats, ceramic paperweights, and handpainted greeting cards.

But for all the winking lights and good cheer, holiday fairs like this one, held at the First Parish Unitarian Church, can be make or break for local crafters. Behind the whimsy is the pressure to sell — now — before the window for ironic onesies and personalized cat toys closes for the season.