When setting up display campaign serving parameters, our preference is usually to allow the campaigns to initially run 24x7x365. We can then use that data to optimize the campaign schedule, as we have measured what times and days of the week are most effective.
In some b2b markets, we will run campaigns on weekdays only and perhaps not overnight. There are some common mistakes to avoid, for example in the US making sure East coast campaigns stay on late enough to shop up for West coast end-of-day business traffic.
The reason for running campaigns (at least initially) 24×7 is that many campaigns perform well at surprising times – days of the week, and hours of the day you would not expect.