If your purpose is to spend your summer holidays with swimming, fishing and other summer joys, it's all in Baikal two months a year, from the beginning of July to the end of August. In June, in the middle of Baikal, it's still quite cold at night, and even the water in the small seas is not yet warming up. In the northern part of the lake, floating ice can still be observed in June.
At the end of August, and later, the wind blows more and more, and the water mixes, causing a significant decrease in its temperature. But if you risk swimming in August-September and not in the closed Gulf, but in Baikal, remember that swimming forever, not because of the cold water (it's not that cold at that time, it's 12-14 degrees). Just remember, I promise...
Beikal is very interesting in the spring from mid-March to mid-April when the freeze and ice travel of Baikal falls short of extremes, and in the fall, from mid-September to mid-October, when the crowds are gone, and the vegetation on the shores acquires a bright autumn colour, the coast of Baikal at that time is just a frame for photographs.