Usually they give birth in June, ... Rattlesnakes hibernating at the same spot in the same crevice. They get out of hibernation from the end of April to the mid of May. Rattlesnake normally breeds every second year. When prey is in especially short supply, snakes may not breed for several years, which is a further example of rattlesnakes' high evolution being well-attuned to their environment.
They are most active in the warmer times of the year—spring through early fall—and many of them are nocturnal during the summer months. When favorable temperatures occur, many rattlesnakes are marginally active even during the winter. You are most likely to see them when the air temperature is between 70° and 90°F (21° to 32°C), regardless of the time of day—be it June or January.
September 20th. On a North facing slope near our home I killed 113. They had moved into prairie dog holes to den and many were giving birth at that time. They do not lay eggs they are born live. These were prairie rattlers in North central Montana. I do not normally kill rattlesnakes I come across, but these were near our farm houses and both I and my dog had been bitten within a week.