To determine if the current date is the first Monday of the month, you can use the following expression:
and(equals(dayOfWeek(utcNow()), 1), greaterOrEquals(int(formatDateTime(utcNow(), 'dd')), 1), lessOrEquals(int(formatDateTime(utcNow(), 'dd')), 7))
The above uses the current UTC date value, so depending on your needs, you can replace utcNow() with the date you need to check or adjust it to your time zone.
I don't understand how you'll use this in a manually triggered flow, but if you wanted a flow to only run on the first Monday of the month, you would create a recurring flow that runs every day and add this expression as a trigger condition.
