With that in mind, I've mined data collected on 104 major online retailers tracked via RetailEmail.Blogspot during a 26-week period (Dec. 30, 2006, through June 29, 2007) to determine the most popular days of the week to send emails.

But before I get into that, I wanted to report the preliminary results of the Email Experience Council survey I mentioned in my column last week on button vs. link calls-to-actions. Seventy-five percent of respondents said that buttons perform better as primary calls-to-action, while only 25% said that links perform better. If you're currently using links, this is some community wisdom worth considering.

So back to choosing the best day of the week. To get an overall sense of the best days, I averaged all the daily "participation" data that I publish each weekend on my blog. From week to week there can be pretty significant shifts in the emailing patterns, but when averaged out, Tuesday was retailers' favorite day to email, followed by Thursday, Monday, Friday, Wednesday, Sunday and Saturday. While 33% of retailers email on Tuesdays, only 8% sent emails on Saturdays.

Looking at general market statistics on open rates and click-throughs, it seems that retailers may be missing an opportunity on Wednesday. According to eROI's "Email Marketing Statistics by Day and Time" study for the second quarter of 2007, which is due to be released within the next few days, the best days for opens are Wednesday, followed by Monday and Thursday, Tuesday and Friday, Sunday, and then Saturday. While 27% of email is opened on Wednesday, only 12% is opened on Saturday.

Click-through rates follow a similar pattern, with 5% clickthroughs on Wednesdays and Thursdays, but only 2% clickthroughs on Saturday and Sunday.

"In prior studies we saw above-average open and click rates on the weekend," said eROI in the report. "However, recently weekends are not performing as they have in the past. This could be seasonal in nature as we typically see dips in performance as the weather gets nicer."

The Effect of Frequency on Day Selection However, those day selections are an aggregate of all 104 retailers regardless of their email frequency -- so it included Apple's twice-a-month cadence as well as Neiman Marcus, which sent me an inbox-crashing 11 emails last week. To generate more meaningful figures, I crunched and sorted the data, aggregating every retailer-week (a week's worth of any retailer's email data) where only one email was sent. I did the same for retailer-weeks where two, three and four emails were sent.

For instance, during the first half of the year, Nordstrom sent from one to three emails a week. Those weeks where Nordstrom sent one email were grouped in one bucket, those weeks with two emails in another bucket, and those with three emails in another. This allows us to see the popularity of each day during any week in which any retailer sent one, two, three or four emails.

When retailers sent one email in a given week, their overwhelming favorite day was Tuesday, with Monday in a distant second place.

However, when retailers sent two emails in a given week, Thursday was their favorite, followed by Tuesday. Wednesday was the least popular workweek day. When retailers sent three emails in a given week, Thursday was the most popular, followed by Monday. Those two days were by far the most popular.

Wednesday's fortunes were much brighter when retailers sent four emails in a given week. While Monday was the most popular, Wednesday was a fairly close second.

While these can make for helpful benchmarks, I urge everyone to test their day selections, as well as their frequency to determine what your particular subscribers respond to best.

This commentary is insightful. I recommend it to others.