There was no Tu BiShvat festival in biblical Israel.
The date developed later as Jewish teachers worked out how the agricultural laws of the Torah should be applied.
For farmers, the new year for trees helped determine the age of fruit and the year to which produce belonged. This mattered for laws concerning tithes and fruit from newly planted trees.
The date fell during the rainy season in the land of Israel. By this point, much of the winter rain had already fallen and a new period of fruit development was beginning.
Ancient Tu BiShvat therefore looked nothing like the modern celebration. There were no commanded festival sacrifices, pilgrimages or special biblical ceremonies. Its original purpose was practical and agricultural.