Thrayodashi is the Sanskrit

Sanskrit

Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 word for "thirteen", and is the thirteenth day in the lunar fortnight (Paksha

Paksha

Paksha , is a fortnight in a month of the Hindu lunar calendar. Literally meaning side or period, that is either side of the Full Moon Day or Purnima...

) of the Hindu calendar

Hindu calendar

The hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. Nepali calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Telugu calendar, Kannada...

. Each month has two Thrayodashi days, being the thirteenth day of the "bright" (Shukla) and of the "dark" (Krishna) fortnights respectively. Thus Thrayodashi occurs on the thirteenth and the twenty-eighth day of each month.

Festivals

• Dhanteras
  Dhanteras
  Dhanteras is the first day of the five-day Diwali Festival as celebrated in parts of north India. The festival, also known as "Dhantrayodashi" or "Dhanvantari Triodasi", falls on the auspicious thirteenth lunar day of Krishna paksha in the Hindu calendar month of Kartik .The Dhan in Dhanteras means...
  
  , also called Dhantrayodasi, is the first day of Diwali
  Diwali
  Diwali or DeepavaliThe name of the festival in various regional languages include:, , , , , , , , , , , , , popularly known as the "festival of lights," is a festival celebrated between mid-October and mid-December for different reasons...
  
  . It occurs on Thrayodashi in the month of Ashvin
  Ashvin
  Ashvin , also known as Aswayuja, is the seventh month of the lunisolar Hindu calendar, the Tamil solar calendar where it is known as Aipassi and the solar India's national civil calendar. It is the sixth month of the solar Bengali calendar. It falls in the season of Shôrot, or Autumn...
  
  .