ואי חוקה היא היכי שרפינן - ... פירש ר"י דתרי גווני חוקה הוו אחד שעושין לשם חוק לעבודת כוכבים ואחד שעושין לשם דעת הבל ושטות שלהם.
...R"Y explains that there are two types of Chukah. One is a Chukah that is a rule for idolatry. One is a Chukah that is made for their own silly reasons.
(א) שלא ללבוש כמלבושי עובדי כוכבים. ובו ג' סעיפים:
אין הולכין בחוקות העובדי כוכבים (ולא מדמין להם) (טור בשם הרמב"ם)... : הגה ..לשם פריצות ... או בדבר שנהגו למנהג ולחוק ואין טעם בדבר דאיכא למיחש ביה משום דרכי האמורי ושיש בו שמץ עבודת כוכבים מאבותיהם אבל דבר שנהגו לתועלת כגון שדרכן שכל מי שהוא רופא מומחה יש לו מלבוש מיוחד שניכר בו שהוא רופא אומן מותר ללובשו וכן שעושין משום כבוד או טעם אחר מותר (מהרי"ק שורש פ"ח) לכן אמרו שורפין על המלכים ואין בו משום דרכי האמורי (ר"ן פ"ק דעבודת כוכבים):
(1) 1. One should not follow the customs of non-Jews (nor should one try to resemble them) (Tur in the name of the Rambam).... RAMA: ...But all of this [i.e., these restrictions] apply only to things that non-Jews do for the sake of licentiousness... [These restrictions also apply] to things that they are accustomed to doing because of a custom or rule that does not have a[ny underlying] reason, out of concern that [these may be sourced in the] “ways of the Amorites,” and that it is tainted by idol worship of their ancestors. But things that they are accustomed to doing for a useful purpose—such as their custom for expert doctors to wear particular clothing so that the doctors will be recognized as specialists—one is permitted to wear [such clothing]. (Maharik Shoresh 88) Similarly, things that are done out of respect or another reason, it is permitted [for one to do such things].
Rabbi M. Feinstein Iggerot Moshe, Yoreh Deah 4:11(3-4):
3. It seams obvious, in my humble opinion, that even in a case where something would be considered a prohibited Gentile custom, if many non-religious people do it for reasons unrelated to their religion or law, but rather because it is pleasurable to them, there is no prohibition of "Do not follow the Gentile custom". So too, it is obvious that if Gentiles were to make a religious law to eat a particular item that is good to eat, Halacha would not prohibit eating that item. So too, any item of pleasure in the world cannot become prohibited merely because Gentiles have turned it into a religious observance.
4. Regarding participating in a Thanksgiving Day honor such as a meal made for the holiday: since there is no religious book that mentions this day as their holiday, nor is there an obligation for this meal, it is rather a national day of remembrance. He too feel joy for the country that has accepted him, or his ancestors, to dwell within, there is not a prohibition to make the joyous meal or to eat turkey.
However, since one may not add to the obligations of the Torah one should not make the traditions a set day and festival.
Encyclopaedia Britannica, 13 Edition:
HALLOWE’EN, or ALL HALLOWS EVE, the name given to the 31st of October as the vigil of Hallowmas or All Saints’ Day. Though now known as little else but the eve of the Christian festival, Hallowe’en and its formerly attendant ceremonies long antedate Christianity. The two chief characteristics of ancient Hallowe’en were the lighting of bonfires and the belief that of all nights in the year this is the one during which ghosts and witches are most likely to wander abroad. Now on or about the 1st of November the Druids held their great autumn festival and lighted fires in honour of the Sun-god in thanksgiving for the harvest. Further, it was a Druidic belief that on the eve of this festival Saman, lord of death, called together the wicked souls that within the past twelve months had been condemned to inhabit the bodies of animals. Thus it is clear that the main celebrations of Hallowe’en were purely Druidical, and this is further proved by the fact that in parts of Ireland the 31st of October was, and even still is, known as Oidhche Shamhna, “Vigil of Saman.” On the Druidic ceremonies were grafted some of the characteristics of the Roman festival in honour of Pomona held about the 1st of November, in which nuts and apples, as representing the winter store of fruits, played an important part. Thus the roasting of nuts and the sport known as “apple-ducking”—attempting to seize with the teeth an apple floating in a tub of water,—were once the universal occupation of the young folk in medieval England on the 31st of October. The custom of lighting Hallowe’en fires survived until recent years in the highlands of Scotland and Wales. In the dying embers it was usual to place as many small stones as there were persons around, and next morning a search was made. If any of the pebbles were displaced it was regarded as certain that the person represented would die within the twelve months.
Encyclopaedia Britannica, Britannica.com
During the Samhain festival the souls of those who had died were believed to return to visit their homes, and those who had died during the year were believed to journey to the otherworld. People set bonfires on hilltops for relighting their hearth fires for the winter and to frighten away evil spirits, and they sometimes wore masks and other disguises to avoid being recognized by the ghosts thought to be present. It was in those ways that beings such as witches, hobgoblins, fairies, and demons came to be associated with the day. The period was also thought to be favourable for divination on matters such as marriage, health, and death. When the Romans conquered the Celts in the 1st century CE, they added their own festivals of Feralia, commemorating the passing of the dead, and of Pomona, the goddess of the harvest.
In the 7th century CE Pope Boniface IV established All Saints’ Day, originally on May 13, and in the following century, perhaps in an effort to supplant the pagan holiday with a Christian observance, it was moved to November 1. The evening before All Saints’ Day became a holy, or hallowed, eve and thus Halloween. By the end of the Middle Ages, the secular and the sacred days had merged. The Reformation essentially put an end to the religious holiday among Protestants, although in Britain especially Halloween continued to be celebrated as a secular holiday. Along with other festivities, the celebration of Halloween was largely forbidden among the early American colonists, although in the 1800s there developed festivals that marked the harvest and incorporated elements of Halloween. When large numbers of immigrants, including the Irish, went to the United States beginning in the mid 19th century, they took their Halloween customs with them, and in the 20th century Halloween became one of the principal U.S. holidays, particularly among children.