HOLIDAYS
Jewish Holidays: What makes them so "High"?
Not all Jewish Holidays are created equal! In fact, it would be hard to answer the question, "Which Jewish holiday is most important?" Is it Shabbat, which comes the most frequently- once a week? Is it Yom Kippur, the holiest day of the year? Is it Passover, which really is the first 'official' Jewish holiday and referred to dozens of times in the Torah?
It is similarly difficult to organize the Jewish holidays: Historically- the ones in Torah (Pesach, Sukkot, Yom Kippur), then other biblical ones (Purim), post biblical (Chanukah), and finally modern (Yom Ha-Shoah and Yom Ha-Atzmaut)? or by category: Pilgrimage festivals: Pesach, Sukkot, and Shavuot, the High Holy Days, Minor Festivals, or (what is usually done): chronologically. And should we start in September with Rosh Hashanah, or follow the biblical chronology of Pesach as the first holiday of the year? Follow the links on the right to explore each of the holidays in depth, and answer the question for yourself! 
The Jewish Holiday Cycle: Or, what's up with all those days off?
The Jewish holidays always fall on their proper dates. Chanukah always begins on the 25th of Kislev; Rosh Hashanah always falls on the 1st of Tishre. Why then does Rosh Hashanah sometimes fall at the beginning of September, and sometimes fall as late as the beginning of October? The Jewish and Julian calendars are not identical.
The Torah's word for month (chodesh) comes from the root 'chadash' (meaning new). The Jewish month follows the lunar cycle and is 29 (or 30) days long. Unlike the Julian or secular calendar (which has 31 day months), where months are no longer bound to their lunar origins, in the Jewish calendar, a new month is always a new moon. Because of this, the full moon always falls on the fifteenth of the Hebrew month. (Note that many Jewish holidays and special dates- Sukkot, Tu B'Shevat, Purim, Passover, Tu B'Av) occur in the middle of the month: on the night with the most available natural light!) Compare this to the Julian calendar where the new moon and full moon have to be graphically indicated as they can move around from the beginning to the end of the month. There can even be (rare but possible) an English month with two full moons, (the second one called a 'blue moon,' heard in the expression 'once in a blue moon' to mean very rarely).
Measuring the months so faithfully according to the lunar cycle creates one small problem. Since many of the Jewish holidays are season-specific (Passover is a holiday of spring) it cannot move more than a few weeks either way, and so the 'lunar' months must also be calibrated to the [solar] seasons. The present Jewish calendar is therefore 'lunisolar,' and adjustments must be made to keep holidays from 'wandering.' (Some lunar calendars live with this. The Moslem calendar, for example, is a wholly lunar year of 354 days. This means that holidays (and birthdays) will slowly migrate across the seasons. The month of Ramadan can occur in any season. Moslem children born in the summer celebrate their 18th birthday in the middle of winter!)
But Jewish holidays, while marking historical events (like the Exodus from Egypt) are also tied to agriculture and the natural cycle. Passover is the holiday of spring. The Jewish calendar's solution is to add a leap month (of Adar) once every three years (approximately) to compensate for the difference; (Actually 9 times in a 17 year cycle). This inserted month is called Adar Beit (Adar II), and when there are two months of Adar, the holiday of Purim is celebrated in the second Adar. This keeps the lunar cycle approximately in sync with the solar cycle, so even though our holidays seem to jump around, they stay fixed in the right seasons.
This extra month of Adar II also changes the number of weeks in a year. Hence, the 54 Torah portions read each week sometimes need to be 'doubled up' in a regular (non-leap) year. These double portions are called 'parashiyot mechubarot.' That's why somethimes two Torah portions are read on Shabbat.