Bob,
For the four years I've been open, I've been trying to get a "handle" on annual buying patterns by asking other framers and other local businesses that are small retail.
I think it is completely volume/"ticket size" related. For businesses that have small ticket sales, the year to year trends seem to be consistent and predictable. For businesses that have "large" ticket sales (I don't have a good number for this, but let's say over $100), then the business has to exceed a certain volume to show trends consistently. (I think the number is about 250k.)
Essentially, while we are small it is much too easy for one or a few sales to send a month in one direction or the other. It's not too surprising when you realize that one sale can represent 10% or more of a month!
Once you reach "trendable" status, WHAT your trends are seems to be HIGHLY location dependent. Mall/weather/tourist area and other factors can drive a particular month consistently up or down.
Just some of my observations and hypothesises. Not truly scientific.
BTW, the summer seems to be the "slow" time for me. But, it could be June, July, or August. When I'm done, the three months seem to be consistent with the same period in the previous year. Usually one of the three months is exceptional, but the other two are down relative to the "average month" for that particular year.
And, what it means is, as I staff up, (this Fall I hope to begin!) I will try to hire "part-timers for "mother's hours" with the understanding that I EXPECT them to take school holidays off, especially the summer!