Shunbun no Hi is the Japanese term for the Spring Equinox, which generally falls on March 20th or 21st. On this year, it is the 20th. Only in 1948 did this day become an official public holiday. Before that, shunki koreisai was practiced. The reason the name and official practice was changed is due to the separation of religion and government outlined in the Japanese Constitution, enforced by Americans after WW2 (a stipulation which Americans have trouble following ourselves, one might ironically note).
"Haru higan" is one of many 'kigo' (season-word) references. This one refers to the week of the Spring Equinox, meaning 'beyond the border of this world' or 'to the other side of the shore'. Haru higan lasts from March 18th to March 24th this year.
The Spring Equinox is a special day, since night and day are the same length. At this time, Buddhists may spend their days visiting graves and soothing the souls of ancestors. Families may visit the graves of ancestors or friends as well during the week of the equinox. Graves will be weeded, new flowers and incense brought, and special treats to be left for the dead to eat may be brought. Usually, some kind of round rice cake or mochi is the gift. For some reason, the spirits of the dead prefer round food. This kind of activity will recur during the Autumn Equinox as well.
The Emperor himself will have koureisai duties to serve to all of the previous Emperors, given that traditionally, Emperors are genetically descended from the Sun Goddess Amaterasu-sama. Shunki koureisai itself was not practiced until the Meiji era, started in 1878 (Meiji 11), probably to further cement public acceptance of his rule.
Other general activities (for farmers especially) include praying for a good harvest during this week, and possibly supplication to village harvest gods. During the autumn, thanks for that years' harvest will be said.
A common saying heard at this time: "Atsusa mo samusa mo o-higan made." Hot and cold will last until Higan!