Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Hong Kong to Burundi as you normally would.
Related info for your journey
Useful info, stats and facts about Hong Kong to Burundi flights.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Hong Kong Intl Airport to Bujumbura Airport flight would often be abbreviated as a HKG to BJM flight.