The Mishnah lists six months for which messengers were sent out for reasons ranging from "major holidays" like Pesach & Sukkos to "minor holidays" like Chanukah. The Gemara then asks about Tammuz & Teves since they have minor fasts, and the conclusion is that during the times of the Beis HaMikdash, those were only optional. But what about Shevat? We learned not that long ago that Tu B'Shevat is the new year for the trees (for maaser purposes, I believe). So wouldn't people need to know exactly when Tu B'Shevat is so that they handle their trees properly? Tithing the crops is a Biblical obligation, so I would think that this would take even higher precedence than say Kislev.
avrohom adler, usa
Tu b'Shevat, as indicated by the Gemara earlier (14a, see also Rashi DH "Hoil"), was not a time when people were harvesting crops, but rather when the new fruits would begin blossoming. If someone had crops, they were generally from the previous year. There was therefore no emergency to know exactly when Tu b'Shevat fell. [In any event, by the time Adar came around, they would know exactly when Tu b'Shevat had been.]
All the best,
Yaakov Montrose