Is this information captured in a "Month" text field or a numerical "Date" field? If it's month, then in "Criteria" of the Query Design, type something like:
[Please enter a month]
What you type inside the bracket doesn't matter, so long as you have the brackets. If this information is in a date format, type something like:
Between [Enter A Start Date] and [Enter An End Date].
Here, the "Between", "And", and the brackets are what's important.