Ramadan is the holiest time in the Muslim faith, similar to a holiday - but at a month long, it would be the longest holiday in existence. Also unlike holidays, Ramadan doesn't commemorate a particular event per se (although Muhammed received his first holy revelation during this time), it's more a time of focus on the Muslim faith, where adherents put extra effort into practicing the tenets of their faith.
Ramadan has no fixed date. The date is determined by a visual sighting of the moon during the ninth month of the lunar Islamic calendar. It begins on Hilal, just after the new moon, which can vary by region and by year - in 2011, Hilal fell in early August; in 2012, it fell toward the end of July.
The name Ramadan is the name for the month between new moon phases, not the name of the event that takes place during that month. To a Muslim, Ramadan is not something you celebrate, necessarily, it's just that you act in a particular way during the month of Ramadan. Although that's not to say there isn't celebration - there is, but not in a 'holiday' sort of way. To Muslims, the entire month is a sacred and joyous time.
Ramadan ends about 30 days later at the start of the next new moon, on Eid ul-Fitr. Adherents are then free to resume normal activities.
Fasting is the most well-known aspect of Ramadan. For the entire month-long period, Muslims are not to eat or drink from sunup to sundown, although exceptions are made for the sick, elderly, children, and pregnant, breastfeeding or menstruating women. But just because exceptions are made doesn't mean they are always taken advantage of. Fasting brings them closer to God, so most people adhere to the fast except in extreme circumstances where it may adversely affect their health.
Ramadan fasting is not dangerous for healthy adults. In fact, studies have shown improvements in blood lipid profiles and positive body composition changes in many adherents. Often, doctors will work with people to keep the fast from interfering with ongoing medical conditions or medications.
Don't they get hungry? Well, yes, at first. But the body adjusts to the fast quickly, and hunger pangs go away. Even during the fast, you are allowed a meal before sunup and one after sundown, although the composition of the meal varies by region and culture.
There's more to Ramadan than fasting - the month is a focus on total spirituality, and as such, Muslims are expected to avoid negative thoughts, words and actions and embody a deeper devotion to the religion.
One of the lessons learned from fasting is deprivation, which inspires generous acts of charity. How nice would it be for everyone, regardless of faith, to take a month to learn what it's like to go without food? How many would come away from the experience with a new empathy for the less fortunate? The Muslim faith places such a strong emphasis on charity that they even distinguish between two different types.
Zakat is a percentage of the income given to the needy. It is a fixed rate, and expected of everyone who makes enough money to sustain themselves and their families. It is such a basic, expected thing that it is one of the pillars of the entire faith. Sadaqa, on the other hand, is a purely voluntary contribution - some practice it always, others practice it exclusively during Ramadan, when the reward for such generosity is thought to be greater.
Even the poor fast during Ramadan, and helping a poor person break their fast at sundown is thought to confer greater reward on both of you. During Ramadan, food donations increase dramatically and public places set up areas for the needy to come break the fast.
The spirit of the whole month of Ramadan is one of sharing, prayer, and holiness. Lights and lanterns are popular decorations, symbolizing enlightenment and the path to God. The Muslim sentiment during Ramadan mirrors the Christian sentiment during the month of December - sure, there's stress and obligation, but the whole thing is so beautiful and so spiritual that you wish it could last forever. Ideally, people from both faiths would strive to carry that spirit with them year-round.