In term of seasonality, both Bafun and Murasaki Uni at actually in season at the same time twice a year around Hokkaido. Uni from the east side of Hokkaido facing the Pacific Ocean are in season during the winter while Uni on the West side facing the Sea of Japan is tin season during the summer.
By region, the season can be broken down to Hidaka in the Spring, Shiribeshi in the Summer, and Spring/Summer for Northern Hokkaido and winter to spring for Eastern Hokkaido. Rishiri Uni from the northside of Hokkaido is only available early June - September.
Most places ban Uni harvesting in October during the spawning season, and since the roe is not very plump yet during November immediately after the spawning season, the season spans from December to the following September.