Adar 15 is "Shushan Purim" -- the day on which Purim is observed in Jerusalem and in other ancient walled cities, in commemoration of the fact that in the ancient walled city of Shushan the original Purim celebration was on this date. (see above, Today in Jewish History).
When Shushan Purim falls on Shabbat -- as it does this years -- a unique phenomenon results: the "Triple" or Three-Day Purim (Purim Meshuleshet). Because a number of the Purim mitzvot cannot be performed on Shabbat, the observances are spread over a period of three days: the megillah reading and Giving to the Poor on Friday; Al Hanissim -- the special Purim addition to the daily prayers and Grace After Meals -- on Shabbat; and Sending Food Gifts to Friends and the Purim meal -- on Sunday. For the details of these laws, see summary and links in yesterday's Laws & Customs.
(The Three-Day Purim phenomenon is unique to Shushan Purim, since the regular Purim -- Adar 14 -- cannot fall on Shabbat in the present-day configuration of Jewish calendar.)