A Year and a Day

"''I'll sit and mourn all at her grave

''For a twelvemonth and a day.

''The twelvemonth and a day being up,

''The dead began to speak:

''"Oh who sits weeping on my grave,

And will not let me sleep?""

- "The Unquiet Grave", Child Ballad #78

The time span of a year and a day has many situations -- legal and magical -- where it is significant. For one thing, it sounds impressive, being more sonorous than merely a year. For another, it's been used in Real Life.

Compare Celestial Deadline, When the Planets Align. This can start or end at a significant time, but it doesn't need to--only the passage of time counts.

Ballads

 * In The Unquiet Grave, the dead lover objects to mourning that lasts any longer.
 * "All Around My Hat" is another folk song that uses this motif. The singer promises to wear a willow around his hat for a year and a day in memory of his lover who's been transported (presumably to Australia).

Film
""In one year and one day, you will be dead!""
 * Equinox

Literature

 * In Susan Dexter's The Wind-Witch, the widow must hold her husband's lands for a year and a day to claim them.
 * In L. Jagi Lamplighter's Prospero Regained, Lilith tells Miranda that freeing the elves from their oaths to Hell means nothing if the elf does not lay claim to his freedom within a year and a day.
 * It also takes a Handmaiden a year and a day to get the Water of Life from a well.
 * In Septimus Heap, a ghost must remain in the place they died for a year and a day. Many other deadlines last a year and a day as well.
 * In the Arthurian Legend tale Gawain And The Green Knight, this is the length of time that passes between Gawain chopping the Green Knight's head off and when he has to go let the Green Knight chop off his head in turn. Also, in some versions of the Quest for the Holy Grail, the Knights of the Round Table agree to search for it for a year and a day, though some take a lot longer getting home.
 * In the Aiel civilisation of The Wheel of Time series, a warrior who was captured would become gai'shain, basically a servant, for a year and a day.
 * In Dark Lord of Derkholm, the Elf Talithan offers to serve Derk for a year and a day in exchange for one of the winged horses the latter breeds.
 * In "The Owl and the Pussycat" by Edward Lear, the title characters "sailed away for a year and a day / to the land where the bong tree grows".
 * In the comic fairy story "Prince Delightful and the Flameless Dragon" by Isaac Asimov, Prince Delightful goes a-travelling for a year and a day, and it's lampshaded that in this fairy story universe journeys always last a year and a day.
 * In Jonathan Strange and Mr. Norrell one of the footnoted stories describes the Raven King disappearing for a year and a day. It's very much in keeping with the magical, mythic atmosphere around him. It causes much worry, because the King told only one person that he was going, and that person misheard him and reported that would return in a day.
 * Ranger's Apprentice: Halt is banished for this period of time.
 * The original Where the Wild Things Are sort of has this. Max's return voyage is "over a year", "in and out of weeks", and "through a day".
 * In The Kingkiller Chronicle, Kvothe is banned from the University's Archives. When Elodin tells Lorren, the archivist, to reinstate him, Lorren says he planned to lift the ban after a year and a day, to which Elodin snarks "very traditional."
 * The Wife of Bath's Tale in The Canterbury Tales. After raping a maiden, the knight is given a year and a day to prove to the court that he doesn't deserve to be executed by learning what it is that women want most.
 * Mentioned quite a bit in The Name of the Wind and The Wise Man's Fear, where it is explicitly invoked as characters trying to sound like they're in a storybook.

Tabletop RPG

 * Earthdawn was in love with this trope and wanted to have its children.
 * Nearly all uses of Blood Magic last for a year and a day.
 * If someone suffers from Depatterning damage to their True Pattern due to Blood Magic use, in order to heal the damage they must refrain from any Blood Magic use for a year and a day.
 * Blood Magic can be used to extend the duration of certain spells to a year and a day, such as False Enchantment, Flying Carpet, Light and Sky Lattice.
 * A character must use a blood magic charm within a year and a day after putting it on his body or it becomes inert.
 * A year and a day after a Black Blood charm is put on, it reaches full strength and the wound it inflicts becomes permanent.
 * A Crystal Limb must be taken to a Weaponsmith once every year and a day for recharging.
 * If a character breaks a Training Pledge Oath they must wait a year and a day to heal the oathbreaker marks.
 * Horrors
 * A Horror Mark lasts for a year and a day. At the end of that time the Horror can renew the Mark for the same period of time.
 * When a Horror creates a cadaver man, if it takes a point of damage the cadaver man will exist for a year and a day instead of the normal three days.
 * The Horror power Thought Worm lasts for a year and a day.
 * The Horror power Unnatural Life only works on a corpse dead for less than a year and a day.
 * The benefits from a number of Talents last for a year and a day, including Book Memory, Empathic Sense and Lasting Impression.
 * Pattern Items
 * If a Core Pattern Item is destroyed, it will be automatically recreated after a period of time, which can be up to a year and a day.
 * In order to become a Core Pattern Item, a Major Pattern Item must have existed for at least a year and a day.
 * Spells
 * The damage the caster takes from casting a Horror Call spell cannot be healed for a year and a day.
 * Dungeons and Dragons
 * 3rd Edition
 * If a paladin's mount is killed, the paladin can call a new mount after this period.
 * Slain or dismissed familiars and Blackguard pets may be replaced after this time.
 * If a shadowdancer's shadow companion is dismissed or destroyed, the shadowdancer cannot replace it for a year and a day.
 * Supplement Relics & Rituals. The warrior-wizard Vode Nulan took a year and a day to forge the magic sword Wicked's Edge.
 * A deity must work for this long to change the traits of his domain and purifying an area of taint using the optional rules for taint.
 * Hot Chicks RPG. The Bind Spirit spell will bind a spirit into the service of the caster. The default period is a year and a day, but the caster can choose a shorter time.
 * In Changeling: The Lost, a year and a day is used as a duration on some particularly potent pledges. A Year and a Day is used instead of a year because of problems with mortal timekeeping (leap years, for instance) synching up with the actual passage of a year, causing the magic to unravel.
 * Time Lord the RPG (based on Doctor Who), Journies supplement. The sample companion Antonio Garcia was challenged to a card game by the Time Lord "The Professor". If Antonio won he would receive The Professor's emerald ring. If he lost he would owe The Professor his servitude for a year and a day.
 * Witch Hunter: The Invisible World. If a shaman casts a Spirit Father spell to bind a spirit as an ally/slave and fails, he cannot try to bind the spirit again for a year and a day unless he knows the spirit's True Name.

Real Life

 * A year and a day has been used in many legal situations.
 * In common law countries, it was held until quite recently that a death could not be murder if the victim died more than a year and a day after the events that the death was attributed to.
 * It is the minimum sentence a felon can receive in some jurisdictions.
 * In medieval Europe, serfs who lived in a free city (or at least avoided being found and brought back to their lord's estate) for this period became free. Among other literary references, this is noted in the Brother Cadfael short story "The Price of Light".
 * The Other Wiki has some more Real Life examples of A Year And A Day.