Why are some holidays not a set date, for example, Thanksgiving, and some are, such as Halloween?

Hallowe'en is more formally known as All Hallow's Eve, which is the day before All Hallows... and that is immovably set to November 1.

https://en.wikipedia.org/wiki/All_Saints

On the other hand, Thanksgiving IS given a "set" time to be celebrated... In the United States, it is the "fourth Thursday" of November...

https://en.wikipedia.org/wiki/Thanksgiving_(United_States)

In Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon date of the year, which is established using an ecclesiastical calendar ... "the Paschal Full Moon is the first Ecclesiastical Full Moon date after March 20 (which happened to be the vernal equinox date in 325 AD)."

http://christianity.about.com/od/faqhelpdesk/qt/whyeasterchange.htm

Easter's date changes every year.

http://www.assa.org.au/edm.html


It is not a particular day, like December 25, but based on the moon's apperance at night. Easter occures during the Jewish feast of Passover.

There are a lot more holidays around the world, some whose date is set with reference to a particular date (Australians celebrate Australia Day on January 26 because that specific date has historical significance), some with reference to a time of the year or month (Australians celebrate Mother's Day on the second Sunday in May, other countries use different definitions for the date of this holiday), some with reference to a solar or lunar calandar (Muslims celebrate Ramadan in the ninth month of the Islamic calendar, which is a lunar calendar). It just depends on how the holiday is originally defined. (And some change - I remember celebrating Washington's Birthday on February 22; then having it change to the third Monday in February, then change its name to Presidents' Day).

https://www.australia.com/en-us/events/arts-culture-and-music/australia-day.html
https://en.wikipedia.org/wiki/Mother%27s_Day
https://en.wikipedia.org/wiki/Ramadan
https://en.wikipedia.org/wiki/Washington%27s_Birthday

One former popular holiday, in the US, has ceased to exist.

Armistice Day, November 11, celebrated the end of WWI.

Now, on November 11, the US celebrates Veterans Day.

https://en.wikipedia.org/wiki/Armistice_Day