March 20th marks the Vernal Equinox. It's one of two points on our calendar when day and night are of equal length. More or less. It may be more of a convenient handle we put on an arbitrary point on our annual revolution around the sun, but it is significant in that it marks the point in the year where we start seeing more daylight than darkness. So with the days growing longer, this is a great time to talk about photoperiod.