problem with javascript and reload page

Question

i have two problem

1. how to call a javascript function from form page and also how to call c# function from javascript because when user click on button i want to validate field of form and if validation is correct then call to c# function .
please any no about this reply

2. in second i have button when i click on button a calender shown and i collect date but problem is when i click on button i page reload , i want to stop reloading of page

in coding i set the value of calender visible true
please any body no about answer reply me