Hinduism almost certainly has a longer list of festivals than any other religious tradition, and there are considerable regional and denominational variations.
Festivals are generally times for celebration and remembrance. Other purposes are:

To create a special atmosphere, diverting the mind from worldly concerns and joyfully focusing on spiritual matters.
To invoke the soul’s natural qualities by creating an environment replete with auspiciousness and the abundant gifts of nature.
To give people spiritual impetus and inspiration, which helps them perform their daily duties.
To dovetail the natural tendency for celebration with spiritual goals.
To forge a healthy sense of belonging by peacefully bringing together individuals, families and communities.