Lookups spiked on March 20, 2013.
March 20 was the first day of spring (in the northern hemisphere) - a day known as the vernal equinox.
Vernal, meaning "of, relating to, or occurring in the spring," comes from the Latin word for spring, vernalis.
Equinox comes from the Latin word for "equal night." On two days each year - the vernal equinox, and also the autumnal equinox that marks the beginning of fall - the Sun is exactly above the equator, and the 24 hours divide equally between day and night.