Bar Kokhba Requests the Lulav and Etrog to Celebrate Sukkot
The vast majority of Bar Kokhba's army was Torah observant. This is evident in many of the writings that were found. In one of the letters, Bar Kokhba requests the Four Species or "Arba Minim" for the waiving of the Lulav during the Jewish Festival of Sukkot. In another letter, the Sabbath is mentioned.

Letter in which Bar Kokhba requests Lulav for Sukkot
Bar Kokhva Letter The Documents from the Bar-Kokhba Period in the Cave of Letters. Hebrew, Aramaic and Nabatean Aramaic Papyri, Y. Yadin, J.C. Greenfield, A. Yardeni, B.A. Levine (eds.), (Jerusalem, 2002). Israel Exploration Society
Overview
Bar Kokhba Letter (P. Yadin 57):
“and send to the camp, towards you, palm branches [lulavin] and citrons [ethrogin]. And you, from your place, send others who will bring you myrtles [hadasin] and willows [aravin].”
In the Bible
"And you shall take you on the first day the fruit (etrog) of goodly trees, branches of palm trees, boughs of thick trees (myrtle) and willows of the brook and you shall rejoice before the L-rd your G-d for seven days...You shall dwell in

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