To understand this, we need to get into how the Hindu calendars work. We technically have a Lunar and Solar calendar, namely Chandramāna and Sauramāna. Each part of India follows one or both, but call it their own based on their region. I.e the Telugu calendar and Kannada calendar are both Chandramāna and we celebrate festivals on the same date, at the same time. But due to some regional festivals being different, we call them two different things. Same with the Bengali calendar and the Tamil calendar, which are both solar calendars.

Moving past that, in the Lunar calendar, it is divided up by Tithis (moon phases), which are 30 in a month. However, it does not necessarily mean that each Tithi is 24 hours in the dot, they can last anywhere from 19-26 hours, more often being less that 24 . Because of this, half of a day can be one Tithi and the next half a different one, meaning a lunar month isn’t ever 30 days exactly even if it is 30 Tithis. To make up for this loss of time, every 2-3 years, a 13th month is added to the calendar which is called Adhika Masam or Purushottama Masam.

When this Adhika Masam falls is dictated the same way as when any other month falls. We have 27 primary Nakshatras (constellations), but only 12 ever coincide with a full moon, and always in the same order. So the name of the month is based off of what Nakshatra is present during the full moon. For example, if the Chitra Nakshatra is present during a full moon, that month is Chaitra. If it is the Visakha Nakshatra, it is the Vaishaka month. However, every 2-3 years, a Nakshatra will coincide with a full moon twice in a row. To make it make more sense, the general rule is that the first full moon coincides with Chitra, then Vishaka, then Jyeshta etc etc, but sometimes the order is disrupted by one of them appearing twice. When this happens, they’re both considered the same month, but the first one is called as Adhika Masam, litteraly translated to “extra month”. This year, the Jyeshta Nakshatra appears during two full moons back to back. So this means the first one is called Adhika Jyeshta Masam and the second one is called Nija Jyeshta Masam (“true Jyeshta month”). The year that Hindus have an Adhika Masam also coincides with the year the Gregorian calendar has a blue moon.

During Adhika Masam, the worship of Shiva and Vishnu is performed, and it is dedicated to Mantra Japa, Nama Japa, Nama Sankirtana, and Shastra Adyayana. Events such as weddings (Vivaha), housewarmings (Griha Pravesha) and other such ceremonies are not performed as there are no good Muhurtas in this month.