Adhik Maas
What it is
The Hindu calendar follows the moon, and a lunar year runs about eleven days shorter than the solar year the seasons actually follow. Left alone, festivals would slowly drift through the seasons — Holi eventually landing in monsoon, Diwali in summer. Adhik Maas (also called Purushottam Maas or the "extra month") is how the calendar corrects that drift: roughly once every three years, an entire extra lunar month is inserted, bringing the lunar and solar calendars back into step. Its start (Prarambh) and end (Samapt, marked by the Amaavas that closes it) both appear on the calendar as their own dates.
Because it is an inserted month rather than a named month of its own, any Ekadashi that falls within it takes the name Kamala (Padmini and Parama) Ekadashi rather than the name it would otherwise carry.
Why we observe it
Adhik Maas is traditionally called Purushottam Maas — the month of the Supreme Person — because in the Bhagavad Gita Krishna names Himself Purushottama, and the month with no fixed festival calendar of its own is dedicated wholly to Him instead. For a Pushtimarg Vaishnav this month is not empty; it is exactly the opposite: a stretch of extra days added purely for extra seva and smaran, with none of the usual seasonal utsavs competing for attention.
A Vaishnav can read Adhik Maas as Prabhu's own generosity: the universe itself making room for more devotion when the ordinary calendar has run its course. Extra reading of the Bhagavat, extra kirtan, extra remembrance through this bonus month are traditionally considered especially fruitful, precisely because the month exists for no reason other than to be filled with Him.