Shalom Chaim,

It's great to hear from you! These are excellent questions.

1. What is the reason that Erev Pesach is the time for Bi'ur?

a. The Jewish calendar aligns tithing with the natural crop cycles in Israel, which has rainy winters for planting and growth, dry summers for ripening and harvest, and transitions in spring and fall. Therefore, Pesach serves as a natural end-of-season, because it is when the old crop cycle concludes and the new one begins. This prevents overlap or confusion with tithes from different years.

b. Specifically, grains are planted in Tishrei when the rain starts; they grow slowly through the winter. Fruit trees conclude their season and restart in Shevat. Ripening and early harvest occur in spring around Pesach time.

c. Therefore, spring is a transition time -- since the new barley is ripening, and will begun to be harvested during Pesach. (On the second day of Pesach, the Korban ha'Omer is brought in the Beis ha'Mikdash, which serves to permit the new year's grain.) Wheat, however, will be ready later, around Shavuos.

d. So, by Pesach, most produce from the previous year has been harvested, consumed, sold, or tithed over the winter. Therefore, stored produce must be tithed now, if they were not tithed already.

e. Granted, Pesach is not the end of all harvesting (harvesting actually continues through autumn). But it is the point where the prior cycle's produce is effectively finished, in the sense that it was probably harvested, distributed, and mostly used up.

f. Think about it this way: If Bi'ur were earlier, then some late-ripening fruits might not be ready for tithing yet. And if it were later, in the summer, then it could present a risk of mixing old produce with the new.

2. What is the source that established this, and which Mefarshim discuss this?

See Devarim 26:12 with Rashi.

3. How does this play out in Halachah?

See Shulchan Aruch, Yoreh De'ah 331:140-141, regarding the timing of Viduy Ma'aseros.

Bi'Verachah,

Yishai Rasowsky