Most hardy cacti bloom in spring. Pediocactus simpsonii for instance (our wonderful mountain ball cactus) can open its first flowers in my garden in March. April and May see the peak bloom on various Echinocereus and suchlike. By mid June most cacti are done and seed is ripening. I grant you the cane chollas (Cylindropuntia) often wait till the 4th of July to bloom...
But there are a whole suite of cacti that are apt to bloom any time in the summer or autumn. Plants from West Texas, from the Chihuahuan desert and uplantds generally, will often bloom during the growing season whenever there's been a good rain. One of the loveliest of these is this handball sized cactus from the southern prairies of Texas. I have been shocked to find Coryphantha sulcata to be reliably hardy. At least the strain sold by Bluebird Nursery in Nebraska has proved to be very cold hardy. I believe they trace their ancestry to seed collected by Alplains Seed company , and further selected by Harlan Hamernik, the almost mythical co-founder of that great nursery.
I took this picture this Monday of a colony of these tiny cacti at the Rock Alpine Garden at the Gardens, where they are thriving on the crevice garden at the entrance. I have seen plants of this wonderful and little known cactus sold at several local garden centers over the years--often at very reasonable prices.
A few other Chihuahuan gems that I have had success with include Coryphantha scheerii and Coryphantha echinus, both yellow flowered as well (and closely related to our plant). If you click on their names you will find other blog posts I've written about both of these as well. Echinocactus horizonthalonius bloomed reliably this time of year in Dryland Mesa for years, although it succumbed to this past brutal winter. And of course there are the enormously charismatic and beautiful Astrophytum and Ariocarpus genera--just a tad tender for outdoor cultivation in Colorado, but superb house plants if left outdoors for the summer. These two completely spineless genera of cacti likewise bloom mostly in summer and fall, due to the same summer Mexican monsoonal flow of moisture that has made our gardens lush and green this summer.