Reference for Arakhin 19:43

שנים עשר יום בשנה מכה בחליל וכו':

[one praying individually completes the Hallel] on twenty-one days: the nine days of the Feast [of Tabernacles], the eight days of Hanukkah, the two Festival days of Passover, and the two Festival days of the Feast of Weeks.

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