Let's say you want to remind your clients the day before a payment is due.
You can head into your account settings by clicking your name in the top right, then clicking Account.
From here you can click 'Settings', then 'Notifications'
It's here you can select if you want the system to automatically send an email the day before a payment is due.
Please note this will cue up a message for every unarchived project with an upcoming payment.
You are able to enable/disable these reminders project by project by clicking the 'Gear' symbol on your projects.