It's usually open for tourists between October to June every year.
Other months depends strictly on the weather as monsoons in Uttrakhand are quite rough.
Rainy season it is closed. Even if the weather is bad or it rains heavily in upper north region they won't allow you to get down to the water. I have visited in the first week of June last year. At that time there was raining in upper north and they were announcing on loud speaker to not to get into the water. Next day also it was closed.
Few activities get closed during monsoon like - river rafting, kayaking. As camping in Rishikesh are now upgraded to luxury camps. so you can enjoy camping throughout the year. Check Out all about camping Here - https://campgangavatika.com/ Get All details about all adventure activities in Rishikesh here - https://campgangavatika.com/activities-in-rishikesh.php