The day before Pesah, Nisan 14, was a very busy day at the time of Bet haMiqdash. Hundreds of thousands, (some say “millions” of Yehudim) who had come to Jerusalem from all corners of Israel were preparing themselves to sacrifice qorban pesah (a sheep to be consumed at night). The people gathered in groups of families called “Habura”. Each Habura, which could consist of 50, 60 and up to 100 people, shared the same qorban. The lamb was carried in the afternoon of 14 Nisan to Bet haMiqdash by one or two representatives of each Habura, where the sheep was sacrificed. Then the animal was roasted in one piece. At night, already the 15th of Nisan, the Habura would do the Pesah Seder, more or less as we do today. They would read the Hagada, the Ma Nishtana questions were made, a lot of food was served, including other qorbanot as hagiga, and people would have matsa and maror. The food was eaten while reclining, same with the four cups of wine, and finally after dinner and before midnight, the qorban Pesah was eaten as an afiqoman (last meal, or dessert) among all members of the Habura. Everyone had to eat from this qorban at least a piece the size of an olive. After they eat the qorban pesah, the Birkat Hamazon was recited. The Gemara says that close to midnight, once the Pesah meal was over, people came out to the terraces of Yerushalayim and from there, looking at the Bet haMiqdash which was illuminated by a full moon, they would sing the Hallel (the Psalms, Tehillim, recited as gratitude to G-d for having liberated us from Egypt). The voices of hundreds of thousands of Yehudim singing the Hallel, all at once, filled the city with a unique melody. The Gemara says that those voices were so powerful that (metaphorically) the terraces of the city would “collapse”.
After the destruction of the bet haMiqdash, we cannot sacrifice the qorban pesah anymore. But we still recite the Hallel at the end of the Haggada, after eating the Afiqoman. We also recite the Hallel tonight at the Bet haKeneset . Another beautiful custom, virtually forgotten today, was also done in remembrance of that famous Hallel of the Bet haMiqdash. As we know, nowadays the qorban pesah is symbolized by the matsa (it is prohibited to assign a piece of meat as a representation of qorban Pesah. See Shulhan Arukh, OH 469). For example, at the end of the Seder we eat matsa again, as “dessert or afiqoman” and we say: [We eat this matsa] “in memory of the qorban pesah which was eaten once one was satisfied.” The forgotten custom to which I’m referring now is that for centuries we did not buy the matsot: we baked them at home. Guess when did we made the matsot? On Nisan 14th, after midday (= today at 3.30, 4.00, pm). Exactly when it was customary to sacrifice the qorban pesah. This is the ideal time to prepare matsot (mitsva min hamubhar). Now, while the matsot were prepared on the eve of Pesah, it was customary to “sing” the Hallel! Today, this tradition is virtually lost. Except in some Yemenite and Kurdish families.