Why it works
Mandatory training has a hard compliance deadline, and completion always lags until someone chases. The reminder is the same every year, so a scheduled email opens the window and keeps the deadline visible — no HR spreadsheet of who's still outstanding running in your head.
Make it yours
Send an early nudge a few weeks out, and consider a second, sterner one closer to the deadline (schedule both). Keep the training link stable so the email never needs editing.