Due to multiple factors, expect to potentially pay more when booking in May. For low rates, April can be a great choice for travel to Arlanda. Flights to Arlanda from Belfast City Airport in April are about £207 on average during April, but can be found for as low as £136.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Belfast City Airport to Arlanda as you normally would.