I assume you mean this species? https://www.inaturalist.org/taxa/271451-Bombus-crotchii
You can use the Explore page on iNaturalist to see where it’s been observed. Here’s a tutorial: https://forum.inaturalist.org/t/the-inaturalist-explore-page-an-introduction/12966
And here are observations made in the Pasadena area: https://www.inaturalist.org/observations?locale=en&nelat=34.301832235151714&nelng=-117.8366844449955&place_id=any&preferred_place_id=1&subview=map&swlat=34.00180454000636&swlng=-118.380507687183&taxon_id=271451
I’d say anytime it’s warm enough to fly is a good time to see bumble bees, although they often will be flying during cooler temps than other bees. So, decently sunny, maybe temps in at least the mid to upper 60s F.