To appreciate the Jewish calendar, one must understand two important ideas: the Jewish calendar is intrinsically linked to the Land of Israel (and its climate); and the Jewish month is based on the moon.
The Land (and climate) of Israel We sometimes forget that the weather where we live colours how we understand our world. A trivial example is that while we, in North America, reading a story that takes place in January, automatically picture snowmen, skiing and sitting in front of a fire, a reader in Australia, will imagine summer vacation!
So too in Israel. On Tu Bishevat the almond trees are covered in white (blossoms), and while the trees in Toronto may also be covered in white, there is quite a difference! In Israel, on Tu Bishevat, Spring is around the corner; while in Canada we are still in the deep of winter. It is hard to celebrate the 'birthday of the trees' when we are bundled up with snow boots, mittens and scarves. Since many Jewish holidays have a natural/agricultural element, if one is not in Israel, these celebrations or rituals may be 'out of sync' with the weather outside our window often causing us to feel disconnected to their meaning.
Even the seasons are different; In North America and Europe, we typically divide the year into four seasons: Spring, Summer, Fall and Winter. In Israel, the year is really divided into only two seasons: the rainy season (corresponding to our winter) and the hot, dry season. Even something as basic as 'rain' means something different in Biblical (and even modern Israeli culture) than our associations.