Due to multiple factors, expect to potentially pay more when booking in September. For low rates, February can be a great choice for travel to Guam. Flights to Guam from Las Vegas in February are about $1,842 on average during February, but can be found for as low as $1,611.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Las Vegas to Guam as you normally would.