There's no shortage of "challenges" to choose from these days. A 28-day Squat Challenge, 30-day Plank Challenge, 30-day Triceps Dips Challenge (seriously?). The allure is pretty obvious: it's just 30 days and you're done. Not to mention the often ludicrous claims associated with some of them. (Planking to lose inches from your waist? Gah!)
Challenges of this sort can be useful, if used properly. But there are, as always, caveats.