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.

Hence, from the first day of Passover until Shemini Atzeret, we substitute the passage Morid haTal, "Who brings down dew", for Mashiv haRuach u'Morid haGeshem, "Who causes the wind to blow and the rain to fall", in the second paragraph of the Amidah. Similarly, from the first day of chol hamo'ed in the passage of Barech alenu, the phrase v'Ten Tal u'Matar liVerachah "grant dew and rain for a blessing" is changed to read "grant a blessing."

Why do we stop reciting the prayer for rain on the first day of Passover even though the days of the Festival are still an auspicious time for rain?

The Sages remarked that Israel should always pray that the Festivals be free of rain, for rain spoils the enjoyment of the Festival. Moreover, were it to rain, it would seem that G‑d chose, as it were, not to allow Israel to celebrate in a spirit of happiness.

In some congregations, it is customary to recite liturgical poems relating to the prayer for dew, immediately before the Amidah, rather than within the Amidah.

Additionally, in many congregations it is customary for the cantor to wear a white garment, kittel, as he does on the High Holidays, when reciting the prayers for dew, so as to arouse the Divine mercy which G‑d metes out on Passover for the crops.