May to early October is the strong season because that is when the Wasserspiele in the Bergpark run on Wednesdays, Sundays, and public holidays. Outside that window the gravity-driven water features are off, which removes about half the reason to visit the park.
A documenta year (every five years; the next is 2027) makes the city a different proposition — packed, with a 100-day exhibition that fills hotels months ahead and turns the whole city into an exhibition venue. Christmas markets in December are pleasant. Spring brings the magnolias and chestnut blossom along the Karlsaue.