Delay of one day for all our festivals?


As per kalnirnay calendar the festival of gokulashtami is on monday 22 aug 2011. and as per our knowledge it rains heavily on this day. But I have observed for last few years that it's raining exactly one day earlier.

Is there a mistake in calculation? Are all our festivals delayed by 1 day?
Can any body look into this? Or am I wrong?

Thank you
Geeta B