When is it?

There is no date. She comes the night a tooth goes under the pillow. Most children lose their first around five or six, and the last around twelve.

The feast of Saint Nicholas is 6 December. Children put a shoe out on the evening of the fifth and find something small in it on the sixth.

Pakjesavond, "parcel evening", is 5 December in the Netherlands and Belgium — the main gift night, separate from Christmas Day.

Christmas Eve, 24 December, with presents opened on the twenty-fifth. Many European countries open theirs on the evening of the twenty-fourth instead.

The first arrives on the night of 12 December and one follows each night until Christmas Eve, then they leave one a night in the same order until 6 January.

New Year’s Eve rather than Christmas. Across much of the Slavic world the tree, the feast and the presents belong to the night of 31 December.

She flies on the night of 5 January and stockings are opened on the sixth, the feast of the Epiphany.

It falls on the second new moon after the winter solstice, which puts it between about 21 January and 20 February depending on the year. The festival runs fifteen days and closes with the Lantern Festival.

The fourteenth of February, the feast of Saint Valentine.

Nowruz begins at the precise instant of the spring equinox, usually 20 or 21 March, and families gather at that exact minute. The celebration runs thirteen days.

The seventeenth of March.

Western Easter falls on the first Sunday after the first full moon on or after the March equinox, so between 22 March and 25 April. Orthodox Easter uses the Julian calendar and is often a different Sunday.

Ramadan is the ninth month of the Islamic lunar calendar and starts with the sighting of the new crescent. Because the lunar year is shorter, it shifts roughly eleven days earlier against the Gregorian calendar every year. Start dates can differ by a day between communities, since some follow local sighting and others a calculated calendar — check with your own mosque or community.

Eid al-Fitr falls the day after Ramadan ends, announced by the sighting of the new crescent. Eid al-Adha comes roughly ten weeks later, at the time of the Hajj. Both move about eleven days earlier each year, and the exact day can differ by one between communities.

On the full moon day of the lunar month Vesakha, which usually falls in May and occasionally in early June. Some traditions observe it on a different day, so local practice varies.

The fifteenth day of the eighth lunar month, when the moon is at its fullest — September or the start of October in most years.

On the new moon night of Kartika, which falls in October or November. The festival runs five days, with the main night in the middle. Regional calendars differ slightly.

The thirty-first of October, the eve of All Saints’ Day, growing out of the Gaelic festival of Samhain.

Eight nights beginning on 25 Kislev in the Hebrew calendar, which lands anywhere from late November to late December. A candle is added each night. Jewish days begin at sunset, so the first candle is lit the evening before the date many calendars print.

On the first properly cold morning — whenever that arrives where you live. That is the morning to look at the windows before the sun clears them.

It depends entirely on where you are. Most of the United States starts in August, the United Kingdom in early September, and much of the southern hemisphere in late January or February.