Nissan is the time of year when trees start to blossom (in the Northern hemisphere).
The first time a year that one sees edible fruit trees blossoming one says:
Once the flowers have fallen off and the fruit is visible, one can no longer say this Bracha .
One only makes this Bracha once a year.
It's an ancient custom to provide the needy with money for their Pessach needs, during the early part of Nissan, up and above one's regular charity contributions.
Source: Kitzur Shulchan Aruch, 60:1, Shulchan Aruch Orach Chaim, 226:1, 429:1
Sunday, 2 Nissan 5776