THE FESTIVAL OF PURIM begins tonight at sundown. Arrangements are complicated this year, due to its proximity to the sabbath:

Fast of Esther Preceeds Three-Day Purim in Jerusalem

by Ezra HaLevi

(IsraelNN.com) The Fast of Esther (Ta'anit Esther) begins Thursday morning and ends after the reading of the Scroll of Esther towards the end of evening prayers Thursday night - when the Festival of Purim begins.

The joyous month of Adar takes a "day off" to commemorate the fasting and prayers led by Queen Esther against the genocidal decree of the evil Haman. Jews around the world fast in remembrance of the prayers, fasting and repentance that preceded the Jewish People's miraculous deliverance from Haman's plan to annihilate them some 2,500 years ago.

A number of rallies and vigils this year will parallel the dangers that loomed in the time of Queen Esther to those facing the Jewish people today.

Three-Day Purim
Purim falls on Friday this year, except in Jerusalem, where the holiday will coincide with the Sabbath. However, because of the Sabbath, the mitzvot of giving gifts to the poor and sending food to at least two people cannot be carried out, and the residents of the capital will do so on Sunday, making it a three-day holiday. The Book of Esther is also not read aloud on the Sabbath, and Jerusalem will join the rest of the country in hearing it on Thursday night and Friday morning. In Yaffo (Jaffa), Acco, Tzfat, Hevron and elsewhere, both Shushan and regular Purim are celebrated due to uncertainty about their walled status during the relevant period.

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The business about "walled status" is explained in the first link above (Judaism 101):

Purim is celebrated on the 14th day of Adar, which is usually in March. The 13th of Adar is the day that Haman chose for the extermination of the Jews, and the day that the Jews battled their enemies for their lives. On the day afterwards, the 14th, they celebrated their survival. In cities that were walled in the time of Joshua, Purim is celebrated on the 15th of the month, because the book of Esther says that in Shushan (a walled city), deliverance from the massacre was not complete until the next day. The 15th is referred to as Shushan Purim.

Best wishes to all those celebrating.