For the source text click/tap here: Rosh Hashanah 10
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The years of a tree are also counted from the first of Tishrei. Why is it important to know how old a tree is? - Because for the first three years the fruit of the tree are called "orlah" and cannot be eaten, so we need to know when to count the first year. As it turns out, if the tree is planted at least 30 days before Rosh Hashanah, this is already considered the first full year of its life. Even so, after the the third Rosh Hashanah, the fruit are not permitted immediately, but only after the fifteenth of Shvat, also known as "Tu B'Shvat."
We explore this unusual festival and its sources.