How to keep biryani hot and moist for a long time
A Holding Cabinet

I'm assuming by your supplier being far away, you mean you can only get a delivery once per day. Keeping food overnight and then serving it all day is probably unfeasible. 

While 8 hours might be a stretch, it sounds like your best bet would be a warmer cabinet. Here's an example. They're temperature and humidity controlled.


  The Hatco Flav-R-Savor® Portable Energy Efficient Holding Cabinet is capable of holding an array of hot foods at optimum serving temperatures for extended periods of time, saving you money through increased energy efficiencies, and allows for advanced preparation ahead of peak-serving periods.