Unslaad means unending, which could mean always. Or, you could say pah tiid, meaning "all times."

Laat means last.

Osos tiid means "some times," though that's somewhat vague.

There is no word for "solstice," though I suppose you could say kiin do evgir, "birth of season." You could specify which season, but only for summer (koor), spring (vah), and autumn (gravuun), since there's no canonical word for winter. (I like to say vokoor, "un-summer," to mean winter.) So, you could say kiir do koor to mean "birth of summer," or you could use -se- to create the compound word kiirsekoor. (For more on compound words, click HERE.)

Equinox is a bit different, and would need to be translated into a larger phrase. Let's say, "when day and night are alike," because equinox is when both daytime and nighttime last equally as long. The direct translation for that phrase would be "Fod sul ahrk vulon los ney." I think that works fairly well to get the concept across.

This could be useful for writing some poetry about the seasons! I might have to do that at some point before the summer is done.