Purim customs are Jewish customs

The internet allows anyone to post his/her opinion.  And you can think whatever you want.  But if you are going to declare something as halacha, you better be prepared with real authorities as backup.  I hold that truth to be axiomatic -- that halacha is what it is,  regardless of whether or not it conforms to your idiosyncratic perspective.  What prompted this is reading post from people who declare that those who celebrate Purim with masks and costume are emulating the akum who dons masks for Carnival, a Catholic holiday with pagan origins. So klal Yisrael is maligned in a broad brush stroke for violating halacha based on a theory that ignores the halachic discussions of dressing up on Purim.

Instead of looking at the sources, the people who advance this fallacious position rely on the  post hoc ergo propter  logical fallacy. As Easter is always tied to Pesach in the calendar, the day before Ash Wednesday always come out a week or two before Purim. Ah, if it follows, we, too must be following. The Jews of Italy saw the nonJews having a rollicking good time before the sober time of Lent and decided that they, too, would like to party with a masquerade. And they tacked that on to the holiday of Purim.  So that theory goes. It simply ignores two points -- halacha and history.

In http://www.vosizneias.com/50116/2010/02/25/new-york-purim-costumes-%E2%80%93-a-history-%E2%80%93-reasons-and-origins Rabbi Yair Hoffman explains that this theory of the origins of costumes on Purim was advanced by Moritz Steinschneider (1816-1907) who erred in tracing its roots to Italy when it is documented even earlier in Germany and France, the bastions of Ashkenazic Jewry. As Rabbi Hoffman says, "The origin is a kasher minhag b’Yisroel from German-Jewry."  Those who deride the custom as a copy of a nonJewish festival are simply wrong, as well as ignorant.  

In http://www.yeshiva.org.il/ask/eng/default.aspx?cat=15&page=2&rc=328, Rabb Moshe Leib Halberstadt cites various  references to the custom in halachic contexts:

The earliest sources I found to the costumes custom in various events is from the era of the Rishonim in the 12th century. In Sefer Yereim of Rabbi Eliezer of Mitz, one of the Baalei Hatosfot and the disciple of Rabbeinu Tam. Also in a responsa of the Rambam quoted in Sefer Maaseh Rokech of Rabbi Masud Chai Rokeach.
The earliest source I found that refers explicitly to Purim costumes is from the end of the era of the Rishonim, in the Musar Sefer Even Bochen of Rabbi Klonimus B"r Klonimus of the 13th century.  

Of course, license to dress up is not license for kalus rosh.  As Rabbi Halberstadt  concludes: "you cannot disapprove of such an ancient Jewish custom. Yet it is very important not to cross the fine line of good taste." 

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