There are several options for send ahead notifications. For the most part, they are all system wide, though you can send clients their reminders manually, on an individual level. Once a reminder goes out, it cannot be sent again.
Enable Automated Notifications
Company > General > Features tab
Check the box for "Notifications: Automatically Send Notifications"
Click Save at the bottom of the page.
Automation set Daily Notification Time:
To set what time the Notifications get sent, go to Company > General: Daily Notification Time
Notification Window Start (minutes) The number of minutes added to the start time to calculate the start of the notification window. Negative numbers will subtract from the start time.
Notification Window End (minutes) The number of minutes added to the start time to calculate the end of the notification window.
Click Save General Settings.
System Wide Reminders:
Under Reports, click on Daily Notifications.
From there change the date to when you want to send reminders and click Update.
From there click Send Notifications.
Resending Reminders:
You can individually resent notifications by clicking the "send updated notification" on the Daily Notifications report. It appears after you've been sent them once.