What I usually do is run ads around the clock, and then dig into the data to learn what is working. It is always best to use data to inform your decisions.
If you are asking about the time of year, you can use Google trends to gain insight from search trends.
Most PPC discussions on this forum have nothing to do with AdSense. AdSense is a topic that relates to publishers, not advertisers. This forum (PPC/SEM) has a lot of advertisers, but not many AdSense publishers, unless they also happen to be advertising as well. That is why the answers were from the perspective of an advertiser, not a publisher.