Movies come out on Fridays because it is the end of the work week, people are more likely to head out to movie if they don't have to wake up early for work the next day. Financially, its the best plan, because you have the chance to reach your largest possible audience.
As for Games on Sundays, it is simple an arbitrary date that doesn't conflict with any other "typical release dates" such as DVD's on Tuesdays, and CDs on Saturdays. Because game companies want people focused on their product, they wouldn't want to release it on the same day as a highly anticipated DVD or movie becuase it draws away potential buyers. Sunday/Monday releases give gamers some play time in the hopes that they will hype up the game to friends/ co-workers over the week and thus drum up more sales.
Good Question!