To sign month, form both hands into the ASL number 1 sign, wherein your non-dominant '1' hand is facing out, while your dominant '1' hand is facing toward you.
While holding your non-dominant '1' hand upright, take your dominant '1' hand, lay it horizontally before the other hand, and move it down from the top of your non-dominant index finger till you reach the back of your non-dominant hand. It is as if you are pointing to one month on the grid of your calendar.
Teach your toddler the month sign when they are ready to grasp the concept of times, days, and longer durations. Month can also be taught when you are teaching your toddler when their birthday lands.
Teach your toddler this sign in conjunction with other time and date flash cards, as well as the specific months in a year and days in a week, etc.