On Passover we stop reciting the prayer for rain [in the Amidah], for rains that fall at the end of Nissan are considered to be a bad sign for the world, the time for the grain harvest has begun and rain will cause the sheaves of grain lying in the field to rot, instead , we pray for dew to fall and moisten the crops without harming drying grain. The first day of Passover was destined from Creation as the time for dew to fall.
...We begin saying Morid HaTal at Mussaf on the first day of Pesach. We say VeTein Brachah in the 9th Brachah of Shemoneh Esrei, beginning with Ma’ariv on the Motzaei Yom Tov, Wednesday night, April 20.
There are two reasons mentioned why we begin reciting Morid HaTal on the first day
1. The Beis Yosef cites a Yerushalmi that states that we pray (daven) for Tal (dew)
and not for geshem (rain) because we want the Three Festivals to have pleasant
2. In Pirkei De’Rebbe Eliezer it states that on Pesach, the Heavenly storehouse of
dew is opened.
I find it ironic that in the month that we cease to pray for rain, Governor Perry issued a proclamation for days of prayer for rain.
NOW, THEREFORE, I, RICK PERRY, Governor of Texas, under the authority vested in me by the Constitution and Statutes of the State of Texas, do hereby proclaim the three-day period from Friday, April 22, 2011, to Sunday, April 24, 2011, as Days of Prayer for Rain in the State of Texas.