Year

A year was a unit of time equivalent to one full solar rotation, which differed from planet to planet. (AUDIO: The Resurrection of Mars)

Measurements
On Earth, a year typically lasted 365 days, each 24 hours long, across 12 months. (TV: World War Three, Last of the Time Lords) More specifically, in an Earth-standard Gregorian calendar, a year was 365.25 days long. (PROSE: Instead of You) The Fourth Doctor once said to a human audience that he "would be over 750 years old in your Earth years", suggesting the years of his homeworld had a different length. (PROSE: A Letter from the Doctor)

A Yquatine year consisted of 417 days, divided into ten months of about 42 days each, further divided into four seasons. (PROSE: The Fall of Yquatine)

A night on Darillium lasted 24 Earth years. (TV: The Husbands of River Song)

The Sontarans measured time with Sontaran martial years. (AUDIO: Heroes of Sontar) On Manussa, time was measured with Manussan years. (AUDIO: The Cradle of the Snake, TV: Snakedance)

The Space Year was a standard human dating system used by colonists on Destination, (AUDIO: The Destination Wars) among other worlds. (TV: Genesis of the Daleks)

Annual events
Beltane was a pagan festival which began at midnight on 30 April — that is, the first minute of the first day of May. 1 May itself was May Day. (TV: The Dæmons)

August bank holiday was described by the Seventh Doctor as a "silly pagan festival". (PROSE: Lungbarrow)