The lake water is cold (between 15-21C) in even the hottest months and locals do not consider the season to be “on” until mid June, with the greatest numbers coming July-August. The water is even warmer in September and those in the know stake out a beach claim then, when the lake has the mildest weather of the year (sunny, balmy) and the fewest tourists.
In high season the lake has mostly sunny days with clouds building up on the western horizon, sometimes dumping the lake with a late afternoon shower before clearing for a star-studded display at night. Nights are cool to chilly.
Time your visit to off-peak days (Monday-Thursday) and you will have the greatest pick of beach. The “crush” is Saturdays and Sundays, when advance reservation for overnight is required on the north shore and the Sevan highway all the way to Yerevan seems one long stream of headlights Friday night heading to the lake and another, slower line heading the 67 km downhill on Sunday evenings.