Hey Shiva, Few dates are pre decided ... but not like only one date .. you will be having 3 to 4 dates for the trip you got ... You have to select one of those dates and leave .... Few are like you can decide the date and go ... Hope I was clear ... Happy Travelling Bro!!!
That depends on the Trip you choose, some of them give you the flexibility to choose dates. But they always mention a definite time period between which you can choose any dates.