Tu B'Shevat did start as a tax holiday, as did many of the other Jewish holidays. The more spiritual aspects of the holiday are more recent. Tu B'Shevat is also considered one of the 4 Jewish New Years.
"There are four new year days: the first of Nisan is the new year for reckoning the reigns of kings and the feasts; the first of Elul is the new year for the tithe of cattle; the first of Tishrei is the new year for reckoning of the years and taking stock of human lives; the first of Sh'vat is the new year for the fruit trees. That is according to the school of Shammai; the school of Hillel says, on the 15th of Sh'vat." -- Mishnah Rosh Hashanah
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