So for me, in the Northern hemisphere (where normal people live) the Summer Solstice is the day with the longest period between dawn and dusk. The Winter Solstice is the longest night, as far as periods between when the sun sets and rises again. The Vernal and Fall equinox marks the days when the period between sunrise and sunset are equal to the period between sunset and sunrise (day and night are exactly equal). These are not supernatural, they are not religious, they are not man-made holidays. These are things that happen to us and our planet as it spins around the sun on it's tilted axis.
But it matters. It affects us. It is part of who we are and how we interact with the planet we live on. It is real.