Beis Shammai holds that the New Year for trees is on  Rosh Chodesh (the first day of the new month) of Shevat, while Beis Hillel says that it is on the 15th day of Shevat. The sap of the tree begins to rise in the roots on Rosh Chodesh, but on the 15th we see the first blossoming on the tree. Shammai's opinion corresponds to that which is concealed, the parts of our life where we cannot see Hashem. Beis Hillel believes that we need to have fruits and see Hashem's love for us. The renewal of life takes place as we leave the coldness of the winter. Rosh Chodesh Shevat is the first awakening of Torah SheBa'al Peh (the Oral Law), a time where the Torah began to bare fruits. The purpose of our lives is to recognize how everything comes from Hashem.