What are the four seasons and what is an equinox and a solstice?

There are four corner posts in your outdoor classroom. we have an opportunity to carve something into these posts. i thought the four corners could each represent one of the seasons. if you have a better idea, then great. if you like this idea, then decide what symbol you would like to carve in each post to represent each season. we will spend a few days before we raise the timber frame laying out and carving these symbols. if we have time we can represent the equinox and solstice as well within the frame.