The other day I was reading some advice from Google about spotting traffic drops and one part caught my eye:
Change the date range to include 16 months. This will help you analyze the traffic drop in context and make sure it's not a drop that happens every year as a result of a holiday or a trend.
12 months of data is enough to see many seasonal trends, but Google is recommending an extra four months to make lead-ins/out really clear.
Too often I hear ecommerce stores looking at too narrow of a period for reporting and jump to the wrong conclusions (and throw away good results). That might be from looking at only a week of traffic data or a couple of months of customer behavior data.
It's easy to get confused by noise when you're looking too closely and think something is happening when it's really nothing at all.
That's why Repeat Customer Insights offers a variety of date options in relevant reports. Depending on the metric, month-based report might be okay but some require a year or multiple years of data to draw the conclusion.