40 Days for Life is an international, twice-yearly, 40 day, 12-hour or 24-hour a day prayer vigil in front of an abortion clinic. It is simultaneously held in as many as 179 cities at a time. The event takes place in the spring and in the fall. Participants go by twos or more and stay for one or two hours at a time, praying for an end to abortion.
This prayerful exercise is a form of witnessing to the tragedy of abortion, but it is also a witnessing to the compassion of a God that calls for everyone to be in solidarity with those who are in distress (the mothers who are considering committing abortion) and those who are about to be killed (the unborn in the womb). It is a public witness, too, in that countless passerby are exposed to the truth about abortion, and they realize for perhaps the first time that there is an abortion facility in their midst, and, if anything, are exposed to the issue, which is normally sanitized or ignored altogether in the media.
Besides the prayer component, 40 Days for Life is a way for people who are unable to stand vigil to pray and fast at home for the end of abortion. The elderly, religious communities, and many others are invited to pray during this time.
Thirdly, there is a "street outreach" component to the 40 Days for Life. Participants go singly or by twos door-to-door spreading the message about the 40 Days event and asking about people's opinions on abortion. There are other participants who hand out flyers and speak with members of the public at subway stations and other busy areas.
Finally, 40 Days for Life is an event where Christians of all denominations can join arm-in-arm in defense of the unborn and their mothers.
For more information about the 40 Days for Life, you can also visit the official international 40 Days for Life site.