There are a number of special events which happen during a round. An alert will be sent a few days ahead of the event starting and these are usually scheduled over weekends.
During a Double XP event all level points gained are doubled.
During a Bullet Rush event, the number of bullets stocked in each Bullet Store every hour is doubled.
During a Motor Madness event, the number of vehicles you see on Vehicle Theft increases from 6 to 10.