I'm pretty much aware of Shravan Maas, we don't eat non-veg and alcohol; some people don't even shave but why there is a gap of 15 days in Shravan Maas of North Indians and Maharashtrians?
Each Hindu month consists of one full lunar cycle, either from full moon to full moon, or new moon to new moon, which would be ~ 28 days. The period from the Purnima to Amavasya takes ~14 days, and is called the Shukla-paksha, or the bright half of the month. From the Amavasya to Purnima is another 14 days, which is called the Krishna-paksha, or the dark half of the month.
North Indians follow the Purnimant calendar, ie, the month begins with the Purnima or the full moon, till the next full moon. In the North Indian calendar, the Shukla-paksha of the month comes first, followed by the krishna-paksha. Maharashtrians & South Indians follow an Amavasyant calendar, ie, the month begins from Amavasya or the New Moon, where the krishna-paksha comes first, followed by the shukla-paksha.