Tonight BH we will celebrate the fifteenth day (in Hebrew= tu) of the month of Shebat, the new year - Rosh haShana- of the trees. tu bishbat is considered the new year of the trees. In this date the trees (and more specifically, their fruits) are considered one year older, independently of when they were planted (See ou.org article below for details). This in an important fact for many Mitzvot connected to agriculture, for example, 'orla: the prohibition to eat the fruits from a new tree during the first three years, for which one needs to know the age of the tree (see more examples here ).
How do we celebrate the 15th of Shebat today?
In our community this festival is known as "ILANOT" (=trees). The custom in our community is to eat tonight from the seven fruits or species by which the Land of Israel is praised: "...a land of wheat and barley and grape and fig and pomegranate, a land of olives and honey (=dates)" (Debarim 8:8).
It is a special zekhut then to eat fruits which actually come from the Land of Israel and say berakha for them.
In Iran, our community used to say one berakha for each one of the seven fruits. The fruits were brought to the table in a covered tray, and the berakhot were said uncovering one fruit at a time. Mr. Nassim Bassalian told me that once in America the standard custom of our community is to say just one berakha for the fruits of the tree (ha'etz) and the correspondent berakha for whatever else is eaten from wheat and barley (normally, mezonot).
No special prayers are added to the regular services, and Tachanun is not recited.