when was pre adimmisions opens

Hey,
Your question is bit incomplete and vague.
Kindly mention the name of the college and the course where you wish to seek the admission,so that I can provide you with relevant information.
However,pre admission are generally meant as trial sessions in which you can get to know about a college before taking admission there. You can know about it's infrastructure,faculty etcetera and then choose according to your wish.
Hope this helps.
Good luck.