IMDB.com is a good source. The movies website will usually also have it. Also movie theatres and stores who sell them will as well. (closer to the date)
One could find movie release dates online by looking to various websites that sell movie tickets and give information on movies. Some examples would be the Internet Movie Database (IMDb), Fandango, and Movie Tickets.
I'm not quite sure, but you can find all the international release dates for the twilight movie at www.twilightthemovie.com
Movie The Movie - 2009 was released on: USA: 2009
Release dates for current movies can be found on AMC, IMDb, Cineplex Odeon, Silver City, Redbox, Empire Online, Fandango, Movie Rewind and Coming Soon.
Not a "for sure" release date, although Marvel named several dates, but no movie. All I know is that a Deadpool movie is in the works.
Adderall The Movie - 2015 was released on: USA: 2015
Home Movie - 2014 was released on: USA: 2014
Blue Movie - 1994 was released on: USA: 1994
Tetherball The Movie - 2010 was released on: USA: 2011
Christian Movie - 2015 was released on: USA: 2015
Movie Memories - 1933 was released on: USA: 1933
Movie Memories - 1934 was released on: USA: 1934