This is Holy Thursday, more traditionally known as Maundy Thursday, from the first word of John 13:34,
Mandatum novum do vibis ut diligatis invicem sicut dilexi vos ut et vos diligatis invicem
In hoc cognoscent omnes quia mei discipuli estis si dilectionem habueritis ad invicem
"A new commandment I give unto you: That you love one another, as I have loved you, that you also love one another. By this shall all men know that you are my disciples, if you have love one for another." [John 13:34, 35]
Today, or more especially tonight, the Church celebrates the institution of the Holy Eucharist at Our Lord's last supper with His disciples before His death and resurrection. In emulation of Him, and in obedience to His commandment, the priest will wash the feet of twelve men at tonight's Mass. As Jesus was taken from the midst of his disciples, the Consecrated Host will be taken to the Altar of Repose. While the candles are extinguished and the lights dimmed, we will stay in adoration, leaving to continue our vigil elsewhere.
An old tradition was for those in positions of authority - rulers, nobles, heads of religious houses, etc - to wash with great ceremony the feet of poor people, the number of the washed sometimes being the liturgical twelve, but often determined by the age of the person performing the ablutions. After the foot-washing and drying, there would be further gifts in the form of clothing, food, drink, and purses of money to each.
A modern way to continue this custom is to write an extra check for the collection plate equal to one's years, or to donate cans and packages of food to the St. Vincent de Paul Society or local soup kitchen - again, equal to one's age. It doesn't take much, and the blessings are immense.
In Germany, this is known as Grundonnerstag - Green Thursday. Why is anybody's guess, and there are lots of guesses out there. However, it is customary to have green vegetables today, most especially a green salad. An old superstition says that those who refuse to eat a green salad today are in danger of becoming a donkey [parents are the same the world over when it comes to making children eat healthy food, and I can just hear a good mutti or vati repeating that admonition as they place a plate of green salad before their kinder].
Of course, in the good old days, with few exceptions fresh greens were still a month or two in the future, so eating a plate of green salad or any green vegetable (by this time quite old) would likely be a form of penance.
Much better writers than I explain the ceremonies and customs of the day at Fisheaters and Catholic Culture. Catholic Culture also has activities and suggestions for celebrations in the home, including recipes for a Seder meal.