The eleventh day (ekadashi) of the bright, waxing half of
the lunar month of Jyeshth (May–June) is a religious celebration.
The eleventh-day observances are all devoted to the worship
of Vishnu, the deity.
Most Hindu holidays have mandated ceremonies, which
generally include fasting (upavasa) and devotion, and frequently promise
particular rewards if they are followed faithfully.
This ekadashi has more stringent requirements than the
others.
Not only is all food prohibited, but the individual
performing the ceremony must also refrain from drinking water, hence the term
nirjala, which means "waterless." The fast must endure from sunrise
until sunset.
Because this ekadashi falls during the warmest time of the
year, this will be a difficult endeavor.
The benefits are substantial: Whether or whether they have
performed the ceremonies for the other twenty-four ekadashis throughout the
year, those who complete the vow for this one ekadashi get religious merit for
all twenty-four ekadashis during the year.
Keeping the ekadashi promise is also said to ensure a long
life and the liberation of the soul after death.
You may also want to read more about Hinduism here.
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