ואכלו את הבשר בלילה הזה
שמות יב ח
What
To eat from the Korban Pesach
Why
So that we should forever remember the miracles Hashem did for us In Egypt.
When
On the night of the 15th of Nissan.
Where
In Yerushalayim
How
There are many details about how it is to be eaten.
Who
Males and females.
If you don't do it
Bais Din may force you to do it, as with any other positive commandment.