Remember that in Judaism, the night always precedes the day, so every holiday begins the night before, and ends at nightfall the next day.
The night of Purim we read the Book of Esther in the synagogue immediately after the nighttime prayers. Each time the name of Haman (the bad guy) is mentioned, we make noises, stomp, yell, that sort of thing. We call this practice Klopping Haman — “Hitting Haman.” It is done to remember the commandment of “You shall blot out the name of Amalek.” (Haman was an Amalekite, and that was his real motive for hating Jews. It was an “inherited” racism.)
Afterwards, we go home and eat a festive meal. The custom today is to visit one’s Rabbis or anyone who has taught you Judaism lessons. In many places people (often students in a yeshivah) put on plays, usually funny plays, but not always. In general, the theme is fun and happiness. A lot of singing and dancing goes on, but it is forbidden to act wild or dangerous. This happiness is a requirement by Law, except for people who are, G-d forbid, in mourning.
The next morning, after the morning prayers, we read the Book of Esther again, doing the same thing at the evil Haman’s name. Afterwards, we eat a festive breakfast meal, and begin the day’s other observances. The day’s obligatory services constitute a number of things, including the giving of charity to at least two poor people. This is called Matanos lo’evyonim. It is a requirement and an obligation, and one of the Commandments we are required to fulfill on the day of Purim.
The Law, however, is that we should give to anyone who asks, and we pray that G-d do likewise to us, to give us whatever we ask just for asking. It is known that Purim is the best day for raising charity, because everyone gives what they can and often more.
Also among the day’s obligatory services is the requirement to give at least one friend two food items that are already prepared and are ready to be eaten. This is called Shalach Manos (though to be grammatically correct it should be called Mishlo’ach Manos, but few people bother with grammar these days, so never mind).
You can read about these in the Megillah, the Book of Esther, Chapter 9, verse 22: “…as the days on which the Jews gained relief from their enemies, and the month which had been transformed for them from one of sorrow to gladness, and from mourning to festivity. They were to observe them as days of feasting and gladness, and for sending delicacies to one another, and gifts to poor people.”
We celebrate by having joy and happiness. That’s why we make plays, that’s why we spend the day in never-ending singing and dancing, going around giving our friends and relatives baskets of goodies and sweets, dressing in costumes, cracking jokes, getting tipsy, giving charity, things like that.
The primary Festive Purim Meal is eaten in the late afternoon on Purim, after Minchah (the afternoon prayer). One must eat bread (washing as Jewish Law dictates), at least one cooked food, and drink at least one cup of wine. We call this the Purim Seudah.
The drinking of wine is significant, because many of the events of Purim happened through wine. Ahasueraus got drunk and killed Vashti, paving the way for Esther to be made queen. Esther gave wine to King Ahasueraus and lowered his defenses, which resulted in his killing Haman. Thus the Megillah tells us that we should celebrate by “feasting.” The Hebrew words for “feasting” means bread, cooked food, and drinking wine. The Talmud therefore tells us to drink on Purim “more wine than we usually do.”
During this meal, it is customary to begin the study of the Laws of Passover, which is just one month away.
The day before Purim, the Fast of Esther, we do not eat from morning until after hearing the Megillah at night, because of the fast that Esther ordained.
At the afternoon service before Purim it is customary to give three coins (preferably silver, and preferably coins with the number Ѕ on them) to charity in memory of the three “half-shekels” given to the Temple. In many synagogues the coins are provided as loans. We donate one-and-a-half dollars to the synagogue, to purchase the coins. We then pick up the silver coins so that we can take possession of them, and then donate them back to the synagogue. This is called Mach’tzis Hashekel, The “Half-Shekel.”
It is correct not to engage in business or go to work on Purim, if this is possible.
Before and during Purim we wish each other “ah fraylechin Purim” — a joyful Purim.