This evening marks the beginning of Yom Kippur, which we've come to call The Day of Atonement. Like Rosh HaShannah (New Year's) this Holy Day is commanded of us in the Torah in Vayikra (Leviticus):
"And this shall be to you a law for all time: In the seventh month, on the tenth day of the month, you shall practice self-denial; and you shall do no manner of work, neither the citizen nor the alien who resides among you."
We generally fast from sundown this evening until sundown tomorrow, and spend this time in contemplation and prayer.
If you observe, may your fast be meaningful.
L'shannah tova!