Source: Baker Hughes
From the Page:
Baker Hughes has issued the rotary rig counts as a service to the petroleum industry since 1944, when Hughes Tool Company began weekly counts of U.S. and Canadian drilling activity. Hughes initiated the monthly international rig count in 1975. The North American rig count is released weekly at noon central time on the last day of the work week. The international rig count is released on the fifth working day of each month.