Day Operation
 

I have a question regarding airfields having no night operation facility.If it is a remote field , what is the latest time you would get airborne before sunset in order to cater for inflight emergencies. This is with reference to turbo props operating from remote fields. Is there any law regarding this ?

Same situation with fog. If there is fog that brings visibility below CAT1 minima you will need a takeoff alternate within 1 hour flight time in an engine-out situation. Same goes for the takeoff at sunset. You will need an alternate with night facilities within 1hr flight time of your departure airport.
So if you have this suitable takeoff alternate, I would say you are allowed to take off until sunset.

day operation
 

That makes sense, but I read somewhere that you need to take off, so that you reach the top of climb, in such a way that in case of engine failure or any other emergency you can come back and land 20 minutes after sunset , that is in twilight , so you work backwards and calculate your take off time. Is that right ?

I operate from two non night operations airports with B737.

Our national regulations have a minimum 5 minutes prior to ECT. My company increases that to require the flight to reach MSA and be able to return to departure with 5 minutes spare to ECT.

Our AOC requires that 5 minutes be increased to 30 minutes in mountainous terrain airports.