Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Dallas/Fort Worth Airport to Regina as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, April can be a great choice for travel to Regina. Flights to Regina from Dallas/Fort Worth Airport in April are about $689 on average during April, but can be found for as low as $354.