(CNN) -- Here's a look at what you need to know about Christmas, a popular Christian holiday celebrating the birth of Jesus Christ. In 2013, December 25th falls on a Wednesday.
Facts: Celebrated December 25th in the United States and other countries.
Christmas is celebrated on January 7 in some Eastern Orthodox countries, such as Russia.
The real date of Jesus' birth is not known.
The Christmas story is told primarily in the Gospels of Saint Luke and Saint Matthew in the New Testament.
Considered the most popular Christian observance, Christmas is also celebrated as a secular family holiday.
The word Christmas is derived from the Old English phrase Cristes maesse (Christ's mass).
The tradition of substituting X-mas for Christmas has its origins in the early Christian church. The first letter of Jesus Christ's name is X in the Greek language.
Many of the customs and symbols traditionally associated with Christmas originated with ancient pagan festivals and winter solstice rituals.
The modern Christmas tree (typically evergreen conifers-usually pine, balsam or fir species) originated in Germany in the 16th century and became popular in England by the mid-19th century thanks to Queen Victoria's German husband, Prince Albert.
St. Nicholas, the real person on whom Santa Claus is based, lived in the 4th century AD in the province of Lycia on the southwest coast of Asia Minor.
Cartoonist Thomas Nast is credited with creating the current image of Santa Claus based on his illustrations that began appearing in Harpers Weekly in 1863.
Economic Impact: Sales during the holiday season may comprise 25-40% of a retailer's annual sales. (National Retail Federation)
Expected holiday spending for November and December 2013 is $602.1 billion, according to the National Retail Federation, an increase of 3.9% over 2012.
December 18 and December 19 are expected to be the busiest days for mail and package deliveries respectively, according to the United States Post Office.