Menstruation is the shedding of the inner membrane of the uterus. Most females of certain mammal species, including humans, undergo the process. Human females experience it roughly once a month from puberty until menopause, except when they are pregnant or lactating.
Religion and menstruation
Throughout history, most of the world's religions have been male-centric. Guys write it, generally about guy gods, doing guy things like shooting lightning, beating each other up, and generally either running amok or controlling everyone and everything. Women's issues - nay, women themselves - were and are generally ignored or feared, and in the case of the latter, demonized.
Prehistoric man is thought to have viewed menstruating women as true power and magic, direct from whatever sense of "god" these people had. A woman bleeds, each month, timed with the moon, and does not get sick or die. She can also create life, seemingly all by herself. It was magical and she was honored. Burials from as far back as 70,000 years ago suggest that red ocher was already being associated with blood, and blood with life. Neolithic people carved and drew goddesses who bled and gave birth. What a wonder Woman was. Until men figured out how unimportant women were (or conversely decided they were too important) and took over religion. In most organized religions, women's menstruation was and is impure, dirty, and best to be avoided.
If ever there was evidence that the Abrahamic Holy books were written by men it's this. As far as the writers of the Torah and the Qur'an are concerned menstruation is a bit... er... yucky. When Eve ate the apple, she was cursed by God and forced to bleed once a month. This blood, then, is the man's reminder not of how stupid he was for simply taking the fruit Eve offered, but how evil and tempting and sinful all women are. The Torah emphasizes this long ago sin, by treating menstruation as something ritually impure and unclean even though it is a natural human process that reasonable people understand is simply part of the reproductive cycle and a sign that the woman is still capable of bearing children. Yet Eve's sin remains. A man cannot have sex with a menstruating woman without making him unclean; in fact, simply touching one will make a ritually clean man, unclean. Even sitting where a menstruating women has sat can make him unclean. And when you are ritually unclean, you cannot approach God, in mind or person.
So strong is the prohibition against touching a menstruating woman, that Orthodox Jewish men try to avoid touching any adult woman, though in crowded places that may become impossible. As far as RationalWiki knows, nobody goes so far as to refuse to sit on public seats just in case a menstruating woman sat on them before.
Menstruation is sinful; after menstruation a woman must sacrifice two turtle doves or pigeons to God to make amends for her sin, a sacrifice that is clearly a sin offering.   If she becomes pregnant, giving birth is worse, there must be the same sacrifice but the new mother is unclean for forty days if the child is male, or eighty days if she failed in her primary duty of producing a son. Women with normal reproductive systems inevitably menstruate or become pregnant so a woman cannot avoid that and it is hard to see how she is guilty. Still, the Bronze Age herders and Iron Age farmers who wrote the Old Testament had no sophisticated ideas of moral accountability and ensuring that whatever people do, they cannot avoid being at fault over something is a good way of breaking self-confidence and controlling them.
The Qur'an takes a very similar line. Apparently "It is an illness, so let women alone at such times"  Possibly Muhammad's wives suffered from PMS/PMT. At least it is considered a sickness and not a sin. It is worth noting that Sura 2, unlike the Torah, reminds men that it is a painful time for women, and they should be left alone and a man should not seek sexual favors at that time. Of course, they are still unclean, in pain or not.
Christian fundamentalists who say they take every word of the Bible literally find following all that about menstruation far too inconvenient so they just ignore it or assume it doesn't apply to them.
Oh what a curse being healthy with The Curse can be. Traditionally, women who are on their periods may not do any "upside down" position during yoga sessions. Some yogis insist that women not risk such positions (well, they never say what you might risk - but it's a law, and you are to follow it!). Those yogis are most often male, of course, and the rules go back to times when women and men were often isolated in yoga practice. Today, any given website that focuses on yoga will address this issue; some say "do what you want', some stay traditional.
On the other hand, the very last thing most menstruating women want to do is wear a big yellow flag that says "on the rag" to the other participants by saying "Oh gosh, a hand stand, I think I'll sit this out. Downward dog, too? Oh, I need water".
The more modern "myths"
If you attended high school anytime before the sexual revolution of the 60's and 70's you were likely to hear any number of myths about "The Curse" and what you should do during it.
- Do not wash your hair while on menses; it will make you go bald.
- Do not bathe until the last day of your menses; you will get dry skin.
- Do not swim; your body cannot handle the pressure.
- Dental fillings will fall out (yes, dentists would routinely ask women if it was "those days").
- Do not have sex during your period. You can get any number of horrors, from diseases to muscle weakness.
- Do not exercise during your period. 1) you are too weak, and 2) it will make you BUTCH.
There's also a popular tale that women who live together eventually find their periods synching up.
In the real world
Menstruation is a natural part of a healthy woman's life. She is not dirty. She is not prone to losing her hair. She is able to swim just fine.
Being upside down in "downward facing dog" does not harm her, or help her any more than being right side up in "moon salute" does.
She may get a bit grumpy and cranky, and hormonal causes aside, you try wearing a diaper for 5 days and see how happy you are.
There are no medical reasons why sex during menstruation should be avoided. It does get a bit messy, but a towel or two works just fine.
And no matter how good your marketing scheme is, there is simply no such thing as a "happy period".
- What the Bible says about Menstruation
- The Bible and the Quran agree: Stay away from menstruating women
- Bible Verses about Menstruation
- On the other hand, a pretty good list debunking many of the myths surrounding this subject
- A colourful flow diagram
- There is much debate about how soon the role of sex was understood for the creation of babies. Nine months is a long time to associate one act with its outcome. The other side to the argument is that while for us it's nine months, for rabbits, horses, and dogs, it's far sooner. The one accepted fact is that by the time we started the basics of agrarian societies (7000 BCE give or take) the role of male and female were well understood.
- Leviticus 15:19-24
- Sura 2 (The Cow):222
- I guess this is the flip side to gay boys not having a strong father figure...
- Does menstrual synchrony really exist?, from the Straight Dope in 2002