The Louisiana Purchase was a treaty settled on April 30, 1803, where the United States bought the territory of Louisiana from France.
The United States bought 828,000 square miles for the price of fifteen million dollars, valuing each square mile as eighteen dollars, in what is known as one of the best deals made in the history of the country. The purchase effectively doubled the size of the country.
This is a notable day in the history of the country, but it is not a public holiday.
In the 17th and 18th centuries, France had a few settlements around the region of the Mississipi River Valley. Towards the middle of the 18th century, with settlements now also in New Orleans, the Great Lakes and Montana, France had control over the United States more than any other European country.
With the French and Indian War, however, in 1762 France had to cede Louisiana to Spain, who no longer had as much power in the European political scene, and thus did not do much in terms of developing the territory for the 30 years that it owned it.
Eventually, Spain declared itself an ally to France, and the British cut the country off from America. Thus, in 1801, Spain returned the Louisiana territory to France with Napoleon intending to form a French colonial empire in America. Around this time, Spain also revoked the deal that allowed Americans the use of New Orleans, which hindered the Americans' effort of westward expansion.
So, President Thomas Jefferson sent Robert Livingston and James Monroe to Paris to negotiate with Charles Maurice de Talleyrand for the purchase of New Orleans. However, in April of 1803, the French asked whether the Americans would be interested in buying all of Louisiana instead. This was due to the troubles that France was having with one of their settlements in Saint-Dominique, as well as Napoleon's fear of going into war with the United Kingdom.
Despite some opposition from politicians in the United States, Monroe and Livingston accepted the deal, and negotiations for the purchase of the Louisiana territory went forward. This purported the biggest acquisition of territory in U.S. history. The treaty was then negotiated on April 30, although it wasn't until December of 1803 that the territory was transferred to the United States' authority.
After the Louisiana Purchase
With all restrictions effectively canceled, the United States was free to go forward with its westward expansion and exploration of new territories.
It was also when the famous Lewis and Clark Expedition was commissioned, and explorers Meriwether Lewis and William Clark were sent to explore the new Louisiana territory to see whether it was viable to have settlements there.
Eventually, in 1812, Louisiana became the Union's 18th state.