The Yoga Comics: Guide to rest days

Via The Yoga Comics:

Ashtanga yoga practice helps us to cultivate more awareness and sensitivity within our relationships with the earth, others, and ourselves. One way we grow this balance of gentleness and strength is by giving equal effort to holding on and to letting go to when we do and not do our asana practice. We diligently hold on to our asana practice on most days and we softly let go of our asana practice on rest days.

Some examples of typical rest days are:

• Taking off moon days and Saturday (or Sunday) gives us time to spend with our families.

• Taking off fever days allows us to give our full strength to heal when we are sick.

• Taking off ladies' holidays honors and nurtures our bodies' natural rhythms.

• Taking off 6 weeks - 3 months after giving birth to allow the body to rest, heal and regain strength.

If you are wondering if you should practice, ask your teacher.

Here is also a useful post of the topic of when not to practice:
http://aylibrary.blogspot.com/2012/08/should-i-practice-if-by-elise-espat.html

Cartoon by: Boonchu Tanti
Editors: Jessica Walden and Elise Espat

Should I practice if I am sick?

Yes, practice at home. 
You don't want to get anyone else sick.  Just do what you can.
If you have a fever, rest.

We are, of course, speaking of asana practice.  Yoga is 24/7.

More "should I practice if..." situations covered here.