Once a year, in the month of Nissan, we have the opportunity to say Birkath HaIlanoth (the Blessing of the Trees). It must be recited in front of two fruit trees which have blossomed with the blossoms visible on them.
It is preferable to say them at the first opportunity. Therefore, if the trees have blossomed, it is good to recite the blessing on Rosh Hodesh (the first day of the month of) Nissan. However, this is not always possible and it is often not even an option in many Western countries where the trees blossom later. In any case, the blessing may be recited at any time during the month of Nissan, including Hol HaMo'ed (the intermediate days of the Festival).
On Shabbath and Yom Tob (the first and last days of the holiday), however, Birkath HaIlanoth may not be recited. If it could not be said during the month of Nissan, it should be recited after Nissan, but without Shem Umalchuth (without mentioning G-d's name).
(See Shulhan 'Aruch 226: 1. Kaf Hahayim ibid, Oth 1. Haggadah Orah Hayim, Birkath HaIlanoth, Oth 7).
