diff --git a/hledger/hledger.m4.md b/hledger/hledger.m4.md
index c31931119..4da8872e5 100644
--- a/hledger/hledger.m4.md
+++ b/hledger/hledger.m4.md
@@ -4768,7 +4768,7 @@ Weekly on a custom day:
 
 Monthly on a custom day:
 
-- `every Nth day [of month]`
+- `every Nth day [of month]` (`31st day` will be adjusted to each month's last day)
 - `every Nth WEEKDAYNAME [of month]`
 
 Yearly on a custom day: