A question submitted by Michael Post:
If the choice of Tu B'shevat is because by then most of the rainy season has passed, but everyone holds that seasons are determined by the solar calendar (as in our V'sayn tal u'mater case), shouldn't the date for the new year for the trees be based upon the solar calendar rather than the lunar? Depending on leap years, etc... the date for Tu B'shevat can vary 3 weeks relative to the solar calendar.
Avrohom Adler, usa
Your question seems to be the basis of what R. Yochanan asked R. Yanai on Daf 15a 'Sh'vat de'Chodashim O Sh'vat di'Tekufah?' To which R. Yanai replied 'de'Chodashim'. It seems that, despite the fact that Sh'vat di'Tekufah would have been more accurate (which prompted R. Yochanan's question), Chazal did not want to fix a date that is based on a different calendar (e.g. the twenty-first of December). So they fixed the closest date to that of Sh'vat di'Tekufah, even though they knew that it would be a few days out. I would even venture to suggest that now that Chazal fixed the New Year for trees on Tu bi'Sh'vat, nature conforms with that, and most of the season's rain falls by that time.
Incidentally, I wonder whether the Machlokes between Beis Shamai (the first of Sh'vat) and Beis Hillel (the fifteenth of Sh'vat) is not somehow connected to this discrepancy.
Be'Virchas Kol Tuv,