I'm going to start my answer by giving you some power search tips for Google, because there's such a wealth of information available online if you just know how to search for it. For example, for a question about the history of celebrating Independence Day would you think that it'd be good to constrain your search to just educational sites? Or perhaps just government sites (remember that encompasses the Library of Congress and many other sites beyond just the obvious government agencies)?

You can constrain your searches to either of these areas by learning about the "site:" keyword. At its most common, you can constrain a search to a specific domain by specifying that domain in the search. That's how most people use this feature. For example, want to learn who Ralphie is and why people at CU talk about him all the time? Search for site:colorado.edu history of ralphie. I like this feature because it's often far more accurate - and faster - than the search feature on the individual Web site.

What most people don't realize is that you can also do searches by constraining just the top level domain (or TLD, as online enthusiasts say). That same search for the history of Ralphie becomes more interesting when you use "site:edu" instead of specifying just the University of Colorado, Boulder. (Did you know that Cornell claims to be the inspiration for CU's "Ask Ralphie", for example?)

Now let's talk about Independence Day. A great way to start a search like this is to explore the results you get from the search site:edu history of independence day. I encourage you to also try site:gov history of independence day, because the latter indeed matches an area on the Library of Congress (loc.gov) all about the history of our celebration of Independence Day.

In fact, without even leaving the Google search results page, you can spot the excerpt from the LOC match that says "Congress passed a law making Independence Day a federal holiday on June 28, 1870" which sort of answers your question. Not 1777, that's for sure.

Digging just a bit into the article, you find this:

"Philadelphians marked the first anniversary of American independence with a spontaneous celebration, which is described in a letter by John Adams to Abigail Adams. However, observing Independence Day only became commonplace after the War of 1812. Soon, events such as groundbreaking ceremonies for the Erie Canal and the Baltimore and Ohio Railroad were scheduled to coincide with July 4th festivities."

Generally, I can find just about anything with a few Google searches. I did just that to confirm the date of the signing of the Treaty of Paris, for example. I've already mentioned the "site:" search tip, but let me state another that might not be obvious: use more words in your search, not less. A search for "july 4", for example, will yield less valuable results than "history of independence day us history". You can't use too many words in a search, but you can use too few, with the inevitable result that you get poor matches.

Ready to learn about the history of Halloween, then? Try a search for site:edu history of celebrating halloween or perhaps site:gov rules about halloween costumes and parties or even religious overtones of halloween history. (Note in the last one that you can sprinkle in a few extra words at the end to help narrow down the results.

Finally, remember that holidays might have more than one name (is it July 4th or Independence Day?) and that, as with anything else you find online, take what you read with a grain of salt if it's from an unknown site. I guarantee a fringe fundamentalist religious site with strong evangelical leanings will have a very different history and interpretation of All Hallow's Eve than a pagan community site.