Why it works
People forget they have leave until it's about to expire, then everyone tries to take it at once. A standing reminder spreads time off across the year and signals that rest is genuinely encouraged. The message is the same every time, so a scheduled recurring email carries it without HR having to remember to send it.
Make it yours
Send it quarterly, then more often as year-end approaches if your policy is use-it-or-lose-it. Point people at wherever they actually check their balance so acting on the reminder is one click away.