"Hashem said to Moshe and Aharon in the Land of Egypt, 'This month shall be for you the beginning of the months; it shall be for you the first of the months of the year.' " (Shemot, 12:1-2)
“Rabbi Yehoshua said, ‘In Nissan the world was created. In Nissan the Patriarchs were born, in Nissan the Patriarchs passed on. On Passover Isaac was born… In Nissan they [i.e. the Jewish people] were redeemed, in Nissan they will be redeemed again in the future.”
Tractate Rosh Hashanah 11a
A special mitzvah, which can be fulfilled only once a year, is to recite the berachah ("blessing" or prayer) made upon seeing a fruit tree in bloom: Blessed are you G-d our G-d, king of the universe, who left nothing lacking in His world, and created within it good creatures and good trees with which He gives pleasure to people. Today is the first opportunity to make this blessing, but it can be done anytime during the month of Nissan (referred to by the Torah as "the month of spring" ). Many visit botanical gardens during this time, so as to avail themselves of an opportunity to observe this beautiful mitzvah.