The Day Parts report enables you to see which of your campaigns or ad groups or headlines were the most effective during what time of the day and during what day of the week. This information can be leveraged to effectively schedule ads that generate significant revenue during certain times of the day or week.
In the above screen shot you can see which campaigns were most active during what time and which of these campaigns brought substantial amounts of traffic as well as revenue.
Note that in the above example we are running only one campaign. However, if you have multiple active campaigns, then you will see each of those campaigns in this report.