Today is the day when the Earth's equator passes through the center of the Sun. This occurs twice each year, around 20 March and 23 September. On the day of an equinox, daytime and nighttime are approximately the same length all over the planet. For most people, this is the start of the change of season.
Additionally, the equinox is the perfect day for conducting trade studies in STK. Especially trade studies that deal with lightning concerns. Centering your trade study analysis period on the day of the equinox allows you to remove biases based on seasonal lighting concerns.
For example, say you are conducting a trade study on the amount of imagery a optical imaging satellite constellation can collect in one week. Each satellite can only collect imagery while the ground is in daylight. Conducting the trade study over a week in December will reduce the collection capability in the northern hemisphere. Conducting the trade study on the day that you are asked to look into the analysis will skew the results based on where the sun is on that day. Conducting the trade study over the equinox will find the average amount of imagery collection that constellation can provide.
So, remember to use the equinox as the center point for your trade space analysis when any lightning concerns are present!