Google has data available here on the proportion of emails outbound and inbound from Gmail using TLS encryption. Apart from an overall increasing trend, a striking feature of the inbound data in particular is a regular oscillation. Here is the past year's data for the proportion of encyrpted email arriving in Gmail inboxes:
Downloading the data and splitting it up by day, it's clear that the oscillation is a weekly variation coming from increased usage at the weekend:
The vertical axis here is the proportion of emails using TLS, so 1 would mean all emails used it. The units on the horizontal axis are `week number', with week 1 beginning Wednesday 11th December 2013, when the data set begins (there's some days with missing data, which appear as the gaps in the graph).
There is some weekly variation in the outbound data too, but it's far less obvious, and doesn't seem to follow the same clear weekday/weekend pattern.
There is bound to be some degree of speculation here, but can anyone offer a plausible explanation? There are clearly factors affecting the volume and type of email sent like this (more work-related emailing during the week, or marketing emails targeted at weekends), but it isn't obvious to me why this particular pattern should result.