What is antonym of solstice?

The antonym of solstice depends on which solstice you're referring to:

* Summer Solstice: The antonym is Winter Solstice.

* Winter Solstice: The antonym is Summer Solstice.

Essentially, the antonyms are the two points in the year when the sun is at its furthest north or south from the equator, marking the longest and shortest days of the year respectively.