All flights to/from VIDP/Delhi Airport now need to get slots approved, and for international flights, you can only apply for these up to a maximum of 5 days in advance.
They’re calling these slots “Delhi Arrival Clearance Numbers” (DACN) for arrivals, and “Delhi Departure Clearance Numbers” (DDCN) for departures, and you can apply for them by emailing email@example.com and copying-in firstname.lastname@example.org.
Make sure you put your slot number in Item 18 of your FPL. If you miss your slot time by more than 30 minutes, expect to have to re-apply for a completely new slot.
Also, watch out for long stays – the maximum ground time for everything except scheduled flights is now 3 days, unless you go into a hangar.
Full details of these new rules can be found here.