The longest night for the Northern Hemisphere.
For me, Solstice reminds me that life is cyclical. Not everything is about going forwards and progress. I am reminded by nature that there are times to slow down and go in and there are times to speed up and go out.
This always helps me to remember that all phases are temporary and helps me to flow with each cycle. If I forget this I find myself resisting the natural flow of events due to not understanding the cyclical nature of reality and this never feels good and is ultimately futile.