Within the 8 limbs of Yoga (according to Patanjali's Yoga Sutra), there are five Niyamas, or internal restraints. Each acts as a sort of ethical guideline intended to help guide the practitioner toward the transcendental reality. Saucha, or purity, is about cleanliness, both internal and external.
Saucha is achieved on an internal level through meditation and keeping a positive outlook. When I was a child, my mother would tell me, "You set the mood. You want to be in a bad mood, then I can be in a bad mood, too. Not too fun, right? Let's be in good moods together." I remind myself of this almost every time I feel a cloud settling over my mind. Worrying, being in a bad mood, these things aren't going to help you progress nearly as much as looking for solutions and keeping your gaze moving forward.
On an external level, cleanliness seems pretty straightforward. Sure, you want to bathe on a semi-regular basis. Obviously, we don't want dirty feet tracking mud all over our pretty yoga mats. But beyond that, keeping your body clean in other ways is important, too. Eating and drinking in a healthy manner keeps the body--your temple--running smoothly. Personally, abstaining from any alcohol was an easy way to help ensure I wake up feeling great. For some, simply cutting back may be a more viable option.
Sundays are typically a day for atonement and a chance at redemption. After a weekend of sleeping late, eating poorly at brunch, or maybe partying a bit too much, Sundays are a great opportunity to reset your goals for the coming week. Clean out the refrigerator, do your laundry, prep some healthy snacks for the week. Set yourself up for success!