Is New Year’s considered a Christian Holiday?
The history of New Year’s celebrations date back to the Julian calendar [pre-Christendom], in which it was celebrated as a day of worship of the Greek deity called Janus, hence its name “January”, according to some historians. Later on, in the Gregorian calendar [Christendom], it was celebrated in Europe, and all Christian countries, as a Christian Holiday, commemorating the circumcision and naming of Yoshka. It was customary for the gentiles to receive gifts on this day as a good omen for the coming year. Until this very day, Catholic churches throughout the world hold a New Years mass, which is considered a day of obligation for Catholics. Protestants, however, do not necessarily view it as a day of obligation, do not hold mass, although many hold services on New Years. Practically, today, many gentiles do not affiliate New Year’s with any religious observance, and in fact are not even aware of the above history. It is simply a day to celebrate the start of the new year on the calendar, and make new year resolutions. Nonetheless, being that this holiday of New Years was affiliated with Christianity, which is defined as idolatry, the Poskim therefore discuss how Jews are to intermingle with gentiles on this day.
May one wish others a happy New Years on the 1st of January?
There is no prohibition involved in wishing a happy New Years to a gentile who does not affiliate the day with any Christian connotations or worship of a deity. One is to avoid wishing a happy New Years to a practicing Christian [particularly Catholics, and Lutherans] who believes in the Christian doctrine behind the New Year’s Holiday. However, even in such a case, one may do so in a pressing situation, in order to avoid causing enmity and anti-Semitism. [In general, it is not customary of Jews to wish other Jews a Happy new year on the first of January. However, it is related, that Rebbe Levi Yitzchak of Berditchiv would wish others a Happy new years on the 1st of January. Likewise, the Rebbe once wished a Chassid a happy new years on the morning of January first, in continuation of the tradition from Rebbe Levi Yitzchak. This is based on the verse in psalms 87:6 ““Hashem Yispor Bichsov Amim..”]
May one partake in a new year’s party?
Obviously, a party which is contrary to the spirit of Torah, contains intermingling with the opposite gender, loud music, and drinking, may not to be participated in by any G-d fearing Jew, even if the attendants are Jewish. Even parties which are free of any of the above moral issues, such as a toast held by companies for their employees, one may not participate in them if they involve any Christian related doctrines, and prayers. Furthermore, even if the party does not contain any religious connotation, some Poskim rule it is forbidden to participate in any party of gentiles, even if the matter can lead to enmity, and even if one will have Kosher food, and Mevushal wine served. However, one may be lenient to join a short New Year’s toast [with Kosher Mevushal wine] held by one’s business in their offices, if it does not contain any religious connotation, and is not an actual party.
 Rama 149:12 [in non-censored editions]; Darkei Moshe Haaruch 148:5; Terumos Hadeshen 195
 Rama 149:12 [in non-censored editions]; Darkei Moshe Haaruch 148:5; Terumos Hadeshen 195; recorded in Likkutei sichos 15/554; Shulchan Menachem 3/292
 Michaber 148:5 regarding presents, that they may be given to a gentile on his holiday if he does not worship idolatry; Avoda Zara 65a
 See Michaber 148:9 that one may not greet an idolater on the day of his Holiday unless he sees him outside, in which case he may greet him in a melancholy voice; See Shach 148:7 regarding if this applies only to the word Shalom, or other greetings; However, certainly saying the words “Happy Holiday” is more severe than simply saying good morning, as it gives credence to their idolatry, and hence should only be done for the sake of preventing enmity, as ruled regarding presents in Rama 149/12; Terumos Hadeshen 195
 Rama 149/12; Terumos Hadeshen 195 regarding presents
 The reason: As a) New Years is considered a Christian Holiday. And b) It denies the true New Year which is on Rosh Hashanah. Hence, we do not want to give credence to the Gentile new year. From the letter of the law however, seemingly there is
 Rabbi Sholom Hecht of Hecht’s bookstore on Coney Island in Flatbush, NY, merited to enter the Lubavitcher Rebbe’s inner chamber for “Yechidus.” One time Rabbi Hecht had a Yechidus with the Rebbe on the morning of January 1st. At some point during the Yechidus the Rebbe told him “Happy New Year”. Rabbi Hecht was very surprised. The Rebbe then told him that Rabbi Levi Yitzchok of Berdichev used to tell his congregants “Happy New Year”, and it’s based on Kapitel 87 verse 6 in Tehillim where it says “Hashem Yispor Bichsov Amim..” “Hashem will count in the register of people…” [Translation courtesy of Tehillim Ohel Yosef Yitzchok with English translation, by Kehot] The Rebbe is also recorded to have wished his secretary, Rabbi Nissan Mindel, a happy new Years on the 1st of January. So is also recorded of the Apter Rav, author of Ohev Yisrael, that he would wish a happy new years and bless the Jewish people on this day. The author of Baal Hayeshuos [Zlotchiv] would also bless the Jews with a good year, and would say with a smile that when Hashem sees how the gentile celebrate the new years, and compares it to the Jews, He tears their evil decrees.
 Michaber Y.D. 151:1; Avoda Zara 8
 Taz Y.D. 152:1 rules the prohibition is due to intermarriage; Chesed Leavraham Y.D. 26 in understanding of Taz ibid; Chaim Beyad 29; Darkei Teshuvah 152:1-5
Other opinions: Some Poskim rule the prohibition only applies to joining a party of gentiles with religious affiliation to idolatry, and only if it will not cause enmity. [implication of Derisha 152 who questions to permit due to Eiva; Shach 152:1 and in Nekudos Hakesef ibid; See Chesed Leavraham ibid
 See Michaber Y.D. 114:1 [no prohibition against drinking with gentile outside of area of sale]