Maslenitsa (Russian: Масленица), also known as Butter Week, Pancake week, or Cheesefare Week, is a Russian religious and folk holiday. It is celebrated during the last week before Great Lent — that is, the seventh week before Pascha (Easter). Maslenitsa corresponds to the Western Christian Carnival, except that Orthodox Lent begins on a Monday instead of a Wednesday. The Orthodox date of Easter can differ greatly from the Western Christian date. In 2008, for example, Maslenitsa was celebrated from March 2 to March 8.

Maslenitsa has a dual ancestry: pagan and Christian. In Slavic mythology, Maslenitsa is a sun festival, celebrating the imminent end of the winter.
On the Christian side, Maslenitsa is the last week before the onset of Great Lent. During Maslenitsa week, meat is already forbidden to Orthodox Christians, making it a myasopustnaya nedelya (Russian: мясопустная неделя, English "meat-empty week" or "meat-fast week").

During Lent, meat, fish, dairy products and eggs are forbidden. Furthermore, Lent also excludes parties, secular music, dancing and other distractions from the spiritual life. Thus, Maslenitsa represents the last chance to partake of dairy products and those social activities that are not appropriate during the more prayerful, sober and introspective Lenten season.

The most characteristic food of Maslenitsa is bliny (Russian pancakes), popularly taken to symbolize the sun. Round and golden, they are made from the rich foods still allowed by the Orthodox tradition: butter, eggs, and milk.

Maslenitsa also includes masquerades, snowball fights, sledding, riding on swings and plenty of sleigh rides. In some regions, each day of Maslenitsa had its traditional activity: one day for sleigh-riding, another for the sons-in-law to visit their parents-in-law, another day for visiting the godparents, etc. The mascot of the celebration is usually a brightly dressed straw effigy of Lady Maslenitsa, formerly known as Kostroma.

As the culmination of the celebration, on Sunday evening, Lady Maslenitsa is stripped of her finery and put to the flames of a bonfire. Any remaining blintzes are also thrown on the fire, and Lady Maslenitsa's ashes are buried in the snow (to "fertilize the crops").

The last day of Cheesefare Week is called "Forgiveness Sunday", indicating the desire for God's forgiveness that lies at the heart of Great Lent. At Vespers on Sunday evening, all the people ask forgiveness of one another, and thus Great Lent begins. Another name for Forgiveness Sunday is "Cheesefare Sunday," because for devout Orthodox Christians, it is the last day on which dairy products may be consumed until Pascha. Fish, wine, and olive oil will also be forbidden on most days of Great Lent.

The day following Cheesefare Sunday is called Clean Monday, because everyone has confessed their sins, asked forgiveness, and begun Great Lent with a clean slate.

During Soviet times, Maslenitsa, like all the other religious holidays, was officially not celebrated. However, it was widely observed in families without its religious significance, just as an opportunity to prepare the pancakes with all sorts of fillings and coverings and to eat them with friends. After Perestroika, the outdoor celebrations resumed, although they were seen by some as an artificial restoration of a dead tradition. Many Russians have returned to practicing Christianity, however, so they are reviving the tradition.

Many countries with a significant number of Russian immigrants consider Maslenitsa a suitable occasion to celebrate Russian culture, although the celebrations are usually reduced to one day and may not coincide with the exact date of the religious celebrations.