We have many festivals and you don't need to book a ticket for attending the festivals but make sure to see when those festivals are celebrated in Nepal, the Nepali dates can differ.
Here check this out: https://www.swotahtravel.com/blogs/festivals-you-shouldnt-miss-in-nepal

 

Nepal has a variety of festivals. When I joined an Everest base camp trek I got a chance to see sherpa people festivals. It was beautiful and loved the way they celebrate it

 

Nepal is very rich in Nature and culture so we celebrate many festivals by a different community. Basically, Dashain becomes the main festival and Tihar (Lighting festival) is the second-biggest festival. This is mainly popular in hilly and Terai region but mountain region depends on the places they celebrate a different way festival like Mani Rimdu festival in Everest and Tiji Festival in Upper Mustang. Here are more details on blogs.
https://www.himalayanadventuretreks.com/mani-rimdu-festival