Purim is not observed as mandated in the Bible
I described in my book “Mysteries of Judaism” that no Jewish holiday is observed today as mandated in the Bible. The rabbis changed every one of them. Many of them had to be changed because their basic observance was a sacrifice, and sacrifices were discontinued when the temple was destroyed in 70 CE. An example is Passover which fell on 14 Nisan. Its only observance was the consumption of the Pascal sacrifice. When sacrifices ceased, so too did Passover, but the holiday was so significant to Jewish history, since it recalled the freedom from Egyptian slavery, that Jews added the name to the holiday chag hamatzot, the Feast of Matzot, that began on 15 Nisan. But while Feast of Matzot, occurring on a different day than the biblical Passover is now popularly called Passover, the Jewish prayer book, the siddur, retains it biblical name Feast of Matzot.
Purim was also changed, but not because of the cessation of sacrifices, because the Purim events occurred in Persia where Jewish sacrifices were not offered.
According to chapter 9 of the biblical book Esther, it was Mordecai who established how Purim should be practiced. It was to be observed for two day on “the fourteenth and fifteenth of the month Adar every year.” Jews “should make them days of feasting and gladness, and of sending portions one to another, and gifts to the poor.”
The first change was to diminish the number of days of Purim. Instead of a two day festival the holiday was reduced to a single day. This is 14 Adar, except for cities that were surrounded by walls during the time when the Purim story occurred. Thus Jerusalem observes Purim on 15 Adar.
The second change, one that became the prominent Purim ceremony for many Jews is the requirement to read the book of Esther on the day of Purim. Later, Rabbi Joshua ben Levi, an uber-nationalist, who made a habit of doubling observances; instituted the nighttime Megillah reading on Purim. The book is therefore read twice. Rabbi Joshua did not double other readings.
The third change, observed by many but not all Jews was to precede Purim with a fast day called the Fast of Esther. This fast was instituted to recall the three-day fast that Esther undertook before she went to the king to beg for the existence of her people. The fast is for only a single day not three as Esther’s fast, and it occurs just before Purim although Esther fasted many months earlier.
This new fast involved another change. Previously, Nicanor day was celebrated on the day before Purim, on 13 Adar. It recalled the victory of the Jews against the Syrian Greek general Nicanor in 165 BCE. When the Fast of Esther was first devised it was placed on the day after Purim so as not to conflict with Nicanor Day. Later, it replaced Nicanor Day on 13 Adar.
An additional practice was soon added: celebrating the day by drinking intoxicating drinks until you cannot distinguish between the hero Mordecai and the villain Haman. While this practice, similar to Mardi gras, attracted many Jews, the rabbis tried to stop it; then failing, sought to minimize the drinking.
In short, every Jewish holiday was changed by the rabbis because of changed circumstances. Those who recognize that Judaism today is not Torah Judaism, but Rabbinical Judaism, are correct. But this fact should not diminish observances; that is, all but the excess drinking.
. The book has just been released in Israel and is coming to the US in a month.
 For example, he fasted for Yom Kippur on both the ninth and tenth of Tishrei (Yerushalmi Megillah 70c).
 See the essay “The Untold Story of Tisha b’Av” by Rabbi Evan Hoffman, rabbi of Congregation Anshe Shalom in New Rochelle, New York, im “Mysteries of Judaism.”
 She was also praying for divine help.
 See Meg. Taanit 12, Babylonian Talmud Taanit 18b, and Jerusalem Talmud Taanit 2, 13.
 See Soferim 17.