This can't be done with a plugin in its own. Check this answer. You need to setup a Cron job in order to so.
Anyway, I think Sama55 created a plugin to handle sending emails asynchronously. It uses Cron too, so you can't get rid of it. But at first sight it doesn't cover the scenario of sending the summary.
By the way, if you end up taking a look at this plugin to void actually using Cron I'd advise you against that. Your requirement implies sending a lot of mails at a given time so many user would find themselves firing an event sending many emails to other users and that would negatively impact on their experience in your site.