Digests

Digest are alerts that are grouped together and sent out out at a set interval. This is useful if you have a busy site and a subscriber doesn’t want to be notified immedatiely so can instead choose to receive a single alert detailing all the changes once a day/week/month.
Apart from when they are sent, digest alerts work exactly the same as regular, immediate alerts.
The message body (content) of a digest alert is exactly the same as a regular immediate alert, except each alert is stacked one on top of the other inside of the digest message body.

Form Builder

To allow subscribers to choose when they want to be alerted, you can enable the ‘Show Alert Frequency Options’ option in the Form Builder
This will show a select box on the Content Notify front-end form that will allow the subscriber to choose between Immediately (default option), Daily, Weekly, or Monthly.

Edit Subscriptions

Admins can change a subscription’s digest frequency manually by editing the subscription in the WP Admin.

Reliance on WP Cron

Digest alerts rely on the WP Cron to function normally. The WP Cron relies on regular interactions or visits to your website in order to keep to time and the schedule to execute accordingly.
There is a good primer article on the WP Cron, what it does, how it works, and how you can ensure it works smoothly, as well as troubleshooting steps, on the Better Notifications for WP website (the sister plugin to Content Notify).
You can find this article here: How to Troubleshoot the WordPress Cron

Change Digest Day

The day of the week and month that you’d like to send weekly and monthly digests alerts respectively can be changed in WP Admin > Subscriptions > Settings > Sending.

Changing Digest Time

If you wish to change the time that any cron runs, you’ll need to use a plugin such as WP Crontrol.

Customising the Digest Alert Template

Should you wish to customise the digest alert template, you can find the template file in wp-content/plugins/content-notify/templates/alert-email-digest.php.
Details on what to do with the template file can be found on this support document: Email Templates