This is the first day of Ayyám-i-Há, the "Intercalary Days" observed by Bahá'ís everywhere. Here's some information about Ayyám-i-Há: where it comes from, what it means and how it's celebrated (much of it adapted from relevant entries in Wikipedia):
The Báb (first of the three Central Figures of the Bahá'í Faith, historically speaking) instituted the Badí' ("wondrous", "unique") Calendar in 1847-8. He divided the year into 19 months of 19 days each, with a number intercalary days (normally four, five in a leap year) to allow the calendar to be a solar one, rather than follow the lunar format of the Islamic calendar. The Báb did not, however, specify where in the year these intercalary days should fall. Bahá'u'lláh confirmed and adopted the Badí' calendar in his Most Holy Book in 1873. He fixed the place of the intercalary days between the 18th (penultimate) and 19th (final) months of the calendar from 26 February (technically, from sunset on 25 February) to (sunset on) 1 March. It was Bahá'u'lláh who gave the intercalary days the name "Ayyám-i-Há" or "Days of Há".
The nineteen months of the Badí' Calendar (now more commonly known as the Bahá'í Calendar) are named after an attribute of God (e.g. Splendour, Glory, Beauty, Grandeur, Light, Mercy, Words, Perfection, Names, Might, Will, Knowledge, Power, Speech, Questions, Honour, Sovereignty, Dominion, Loftiness). Ayyám-i-Há symbolises the transcendence of God over these very attributes. "Há" (the Arabic letter corresponding to the English "H") has been used in Bahá'í scripture to represent the unknowable essence of God. Under the Arabic abjad system, the letter Há has the numerical value of five, equal to the maximum number of days in Ayyám-i-Há.
During Ayyám-i-Há, Bahá'ís are especially encouraged to celebrate the unity of God by showing and sharing fellowship. Bahá'ís often give and receive gifts at this time, although there are no set rules about this. It is also a time of charity and goodwill, when Bahá'ís often participate in various projects of a humanitarian nature. It's not as if Bahá'ís are meant to leave off doing such things the rest of the year, but at this time they are spurred on to make special efforts in doing so.
As is the case with virtually every aspect of the Bahá'í teachings and writings, there are levels of deeper mystical and symbolic significance to Ayyám-i-Há, some of which are described or alluded to here. But you don't have to apprehend all the meanings of Ayyám-i-Há in order to understand it or, more importantly, to join in and enjoy it.
Find out more about how Bahá'ís around the world celebrate Ayyám-i-Há on the website of the Bahá'í International Community.
The photo above is from the Garden of Ridván, a place of pilgrimage at the Bahá'í World Centre in the Holy Land. I have a photo of me, from my pilgrimage there in February 1988. It was so cold and windy that day that all the water is coming out only one side of the fountain!