The best ideas often have a retrospective simplicity. What better cure for the Monday blues than a finger-snapping, retro dance party? Such is the logic behind Motown on Mondays (MOM), a dance-floor franchise that started in San Francisco back in 2009 and has since expanded to eight cities, including Oakland, Brooklyn, and Chicago. Its appeal is self-evident: At the Legionnaire Saloon — where gilt-framed portraits of English nobility share wall space with James Brown concert posters — patrons down $5 margaritas and throng the small dance floor, singing along with half-remembered classics like “Going to a Go-Go” and “My Guy.” Later, the old hits give way to uptempo remixes — “Let’s party our faces off!” cries the DJ — and things get politely funky. Lack of space limits most to a spirited two-step, but there are outbreaks of hand-jive. Throughout, the “Mom” acronym proves remarkably apt — this party is steeped in maternal solicitude. Mom knows Mondays can be tough. She’s pulled out her favorite records, and everyone’s going to dance.