Fastelavn is a Carnival tradition in the Northern European, and historically Lutheran, nations of Denmark, Norway, Sweden, Latvia, Estonia, Iceland, Faroe Islands, as well as Greenland.

The traditions of Fastelavn vary somewhat by country and by local region, as some traditions have changed over time. A common theme of Fastelavn in all the countries currently involves children dressing up in costumes, walking door to door while they sing and gathering treats for the Fastelavn feast, a form of trick-or-treating. Today, the festivities of Fastelavn are generally considered to be a time for children’s fun and family games.

Fastelavn is celebrated seven weeks before Easter Sunday and therefore always falls on a Sunday between February 1 and March 7.

The term Fastelavn comes from Old Danish fastelaghen, which was a borrowing of the Middle Low German vastel-avent, meaning "fast-evening", or the day before Lent. The word has cognates in other mostly Germanic languages and languages with contact with it, including Kölsch Fastelovend, Limburgish Vastelaovend, Dutch Vastenavond, Scots Fastens-een, Latvian Vastlāvji, and Estonian Vastlapäev.

The related word Fastelovend is used for Carnival in Germany in Cologne and Bonn with the same meaning. Fastelavn is related to the Roman Catholic tradition of Carnival in the days before Lent, although after Denmark became a Protestant nation the festival adopted certain distinctive characteristics. The holiday occurs the week before the Christian penitential season of Lent, culminating on Shrove Tuesday, the day before Ash Wednesday, the first day of Lent. The Swedish counterpart is Fastlagen, the Icelandic is Sprengidagur, and in Finland they celebrate Laskiainen. In Estonia it is celebrated as Vastlapäev. In Iceland, Ísafjörður is the only town that celebrates Fastelavn on the same day as the other Nordic countries, on Monday, locally known as Maskadagur (mask-day).

As in Carnival traditions elsewhere, dressing up in costumes forms an important part of Fastelavn in all the Nordic Lutheran countries where this festivity is celebrated. In some places this involves smaller processions, but in contrast to former times, dressing up in costumes are now mainly a children's activity only.

In Norway, students having seen celebrations in Paris introduced Carnival processions, masked balls and Carnival balls to Christiana in the 1840s and 1850s. From 1863, the artist federation kunstnerforeningen held annual Carnival balls in the old freemasons lodge, which inspired Johan Svendsen's compositions "Norsk Kunstnerkarneval" and "Karneval in Paris". The following year, Svendsens Festpolonaise was written for the opening procession of the Carnival ball. Edvard Grieg also attended the Carnival, and wrote "aus dem Karneval" (folkelivsbilleder Op. 19). After the Rococo Hall at Grand Hotel opened in 1894, annual balls in the Carnival season were arranged until the hall was destroyed in a fire in 1957. Since 1988, the student organization Tårnseilerne have produced annual masquerade balls in Oslo in the historical renovated freemasons lodge in the Carnival tradition, with masks, costumes and processions after attending an opera performance. The Carnival season also includes Fastelavens søndag (with cream buns) and fastelavensris with decorated branches.^{[citation needed]}

Traditional events include slå katten af tønden ("hit the cat out of the barrel"), which is somewhat similar to using a piñata. It occurs following the church service of Shrove Sunday in parishes of the Evangelical Lutheran Church in Denmark and, also, at non-religious celebrations in city squares etc. This tradition is also celebrated in Scania (Skåne) in southern Sweden, a formerly Danish area. To play this game, a wooden barrel full of candy is strung up in a suitable location and then the game is on: the participants each get a go to strike the barrel with a club or stick. There are sometimes oranges too inside the barrel and it has the image of a cat painted on it. After the candy pours out, the game continues until the entire barrel has been smashed to pieces. The one who knocks down the bottom of the barrel (making all the candy spill out) becomes kattedronning ("queen of cats"); the one who knocks down the last piece of the barrel becomes kattekonge ("king of cats"). In some places, the person behind the kattekonge becomes kattedronning, although this is not common. The same tradition is held in the Faroe Islands but the barrel is most commonly empty. The person to finally knock down the last piece of the barrel becomes kattakongur ("king of cats").