That's your "perfect month" award. You will achieve it by filling the activity (red) ring every day of the month and then the badge will turn gold. It's under the "complete your rings" section.
The monthly challenge badge looks exactly the same when not completed but will turn silver upon completion. It is also a more specific challenge, like walking/running a distance, completing a number of workouts, getting 200% move goal for a number of days etc. This one is under the "Monthly challenge" section, above the "complete your rings" but below the "special challenges" section.