The Book of Esther records that the Jews agreed “to observe them as days of feasting and joy” (Esther 9:22).
One is obligated to partake in a festive meal on Purim day.
The bare minimum to fulfill this mitzvah requires that one wash and eat bread and then recite the bentching, the Grace after Meals.
One should include Al Hanissim, the special prayer for Purim, in bentching.
If one forgots Al Hanissim, one does not repeat bentching.
It is customary to invite guests to one’s Purim meal.
The Purim meal is normally held later in the day so that the feasting and rejoicing carries over past sunset into the next day.
The festive meal concludes with bentching, Grace after Meals, which can be found in any Jewish prayerbook or bentcher, special Grace after Meals booklets.
Al Hanissim is added to bentching before the conclusion of the 2nd blessing.