It is, of course, the planet's equinox (not to be confused with the solstice, which couldn't be more different). The word "equinox" comes from the Latin for "equal" and "night." Essentially, it's the biannual event when the sun is directly over the equator.
Regardless of which side of the planet you live on, Google has you covered with a Doodle that reflects the mood of the season ahead of you. Those witnessing the end of summer in the north see a Doodle featuring a brightly colored leaf resting on our now-tired globe, seemingly reminded of the fleeting warm days as it girds itself for shorter, colder days ahead.
Meanwhile, those welcoming spring in the south see the sprouting of new life from an anthropomorphic globe, perhaps astonished and optimistic about the warmer days and new life ahead.
If you aren't too keen on this situation, remember that you can count on things moving in the opposite direction in six months.
Originally published March 19.
Update, Sept. 23: To reflect Google showing the fall Doodle in the the Northern Hemisphere and spring Doodle in the Southern Hemisphere.