What is the significance of Tu B'Shvat? How was it celebrated through history, and was it celebrated through history? What are the Halachic torah sources for Celebrating it? Why the recent Popularity?
Tu B'Shvat appears first in Mishnayos Rosh HaShana as one of the four Rosh HaShanas. The first of Shevat is the new year for trees, according to Bais Shamai, however according to Bais Hillel the fifteenth of Shevat. Fruit trees use Tu Bishvat as the cut off date in the Hebrew calendar for calculating the age of a fruit bearing tree. The Halachos of Orlah remain to this very day in the same form it had in talmudic times and uses Tu B'Shvat in the same way.
In the 16th century, the Arizal instituted that on Tu Bishvat we eat fruits from trees of Eretz Yisrael. By the Chassidim, some pickle or candy the Esrog from Sukkos and eat it on Tu Bishvat. Some pray that they will be worthy of a beautiful Esrog on the following Sukkos.
also see this link for more information http://www.chabad.org/kabbalah/article_cdo/aid/379846/jewish/Tu-BShevat-Basics.htm