Most holidays acquire meanings. This one has been rebuilt from the foundations three times, and the versions have little to do with each other beyond the date.
The tax date. In the Mishnah it is a boundary in an accounting system. Tithes on produce had to be assigned to a year, fruit could not be tithed from one year's crop against another's, and something had to mark where one year's fruit stopped and the next began. The fifteenth of Shevat is that mark, chosen because by then the winter rains have mostly fallen and the sap is held to have risen. It is a rule about revenue, and it is celebrated by nobody.
The kabbalistic seder. In sixteenth-century Safed, Isaac Luria's circle read the eating of fruit as an act with consequences in the upper worlds. They built a rite modelled on the Passover seder: four cups of wine shading from white to red as the year turns from winter to spring, fruits sorted by whether they have an inedible shell, an inedible pit, or neither, each class mapped onto a level of creation. The earliest printed text, Peri Etz Hadar, appeared at Venice in 1728 with no author's name on it. This is a mystical repair ritual, not an agricultural feast.
The tree-planting day, twice. Zionist settlement from the 1890s made Shevat 15 the day for putting saplings in the ground, and school children in Israel still do. That reading is national and territorial. From the 1970s a fourth layer has been added by Jewish environmental groups who kept the planting and changed its object, so that the day now also carries climate teaching and a critique of consumption.
None of these replaced its predecessor cleanly. All four are currently observed, sometimes in the same building on the same evening.