This question is a little bit broad, but from your examples, let's say you mean holidays in the United States.

There really isn't a specific answer to the question, why a holiday takes place on a certain day is usually pretty specific to the holiday itself. For instance, Christmas is always placed on the same day in the calendar because it is intended to celebrate the birth of Jesus of Nazareth. I won't get into why December 25th in particular was chosen. Halloween is celebrated on the 31st because it is the day before November 1st. See more on that here http://www.reddit.com/r/AskHistorians/comments/1mku30/what_is_the_history_behind_halloween/.

Easter is always on a Sunday because of the christian theology of the death (Good Friday) and resurrection of Jesus (Easter Sunday).

Labor day, Memorial day and Thanksgiving (practiced in the U.S. because other countries celebrate these on entirely different days) were all determined by either Congress and/or state legislatures. They likely decided to make these fall on particular days of the week so that these holidays would fall on convenient days for banks to be closed and for people to be off work (most federal holidays are on Mondays).

Again, no one answer - each celebration has its own history and details that determined when it was celebrated.