dafinch, you are attempting to use digests in a way that it was never intended. It was never designed be turned off on weekends. From the beginning as a phpBB 2.0 mod, digests were intended to be sent once an hour, 7 days a week. As a mod, it required setting up a cron job on your web server calling
mail_digest.php to do this once an hour every day of the week, and only once an hour.
As an extension, it works inside phpBB's cron system. phpBB crons run whenever there is board traffic. When there is board traffic, if an hour has elapsed since digests were last sent out, digests go out. If it has been many hours, many hours of digests will go out. See the first post of the topic.
If you create a system cron to run at least hourly, it still calls phpBB's cron system. So if you disable a cron on your web server on the weekend, a queue will develop. If "down" for two days it will process 48 hours before catching up.
Ideally, don't turn a cron off on weekends so no queue develops. If you insist on doing this anyhow, the best thing to do is to have each user's digest settings tuned so they are not sent a digest if there are no posts, which should not happen on the weekend. UCP > Digests > Additional criteria > Send a digest if there are no new posts = No. Digests will still churn through those 48 hours on Monday, but it should go quickly as there will be nothing to send.
Manually, before you enable the cron on Monday, you can reset the mailer. ACP > Extensions > Digests > Reset mailer. This would clear any backlog.
You need to carefully read the first post of the topic so you understand the design constraints of digests.