Adhik maas: the extra lunar month
Adhik maas is an extra lunar month added about every 32 to 33 months, so that the lunar year stays in step with the solar year.
What it is
A lunar month without a sankranti, when the Sun does not enter a new sign during the month, is the traditional condition for an adhik maas. The extra month is named after the following month in the usual rule.
How it is worked out or read
Because the rule depends on the sankranti dates, the adhik maas is found from the Sun's entry into the signs over the year. The site does not yet list adhik maas dates.
Where it appears on this site
The sankranti and tithi are in the panchang. See sankranti explained for the rule that defines the months.
What an extra month is
An adhik maas is an extra lunar month added about once every two and a half years, so that the lunar year stays close to the solar year. It is inserted when a lunar month passes without the Sun entering a new sign, which is the traditional condition for it.
How the extra month is named
In the usual rule the extra month takes the name of the month that follows it. Festivals set in that name may therefore fall in the extra month or in the regular month, depending on the tradition. A reader should check which month a festival is assigned to in the chosen calendar.
Planning around an adhik maas
Many traditions avoid certain observances in an adhik maas, and others prefer it for particular rites. A planner should check the tradition's rule and note the adhik maas in the year's calendar, since it can change the dates of festivals that follow the lunar months.
Why the calendar needs the extra month
Without an extra month, the twelve lunar months would fall about eleven days short of the solar year each year, and the festivals tied to the lunar months would drift steadily through the seasons. The adhik maas corrects this drift every two and a half years or so, which keeps the lunar calendar broadly in step with the seasons. The correction is simple in principle, and the details are settled by the rule each tradition follows.
Readers who notice a year with an extra month are usually surprised at first, but the mechanism is an ordinary part of a lunisolar calendar, and it is one of the features that makes the traditional calendar workable across many years.
What a planner should record
A planner working across years should record the adhik maas in the calendar, together with the rule used to place it. Festivals that fall in that month may be observed in the extra month or in the regular month, and a written record avoids confusion when the same festival is discussed in a later year.
The opposite case
Very rarely a lunar month contains two sankrantis, and some calendars then drop a month, called kshaya maas. This is far less common than an adhik maas.