Every two (sometimes three) years in Vedik Sidereal astrology there is a time compensation month added to correct, and adjust the time movements. This is something similar to the western Leap Year.
This month is considered to be very auspicious (adhika = best or greatest), as Vishnu Himself appearing in a Calendrical Form to rectify our astrological and astronomical calculations.
The astronomical parameters differ only slightly in different astronomical texts. For our present discussions, these differences will not matter much and so we follow the Suryasiddhanta. According to this text,
1 Lunar year = 29.5305 X 12 = 354.366 days
1 Solar year = 365.2587 days.
Thus one lunar year lags behind the solar year and one lunation has to be added after 3 years to fit the lunar year into the solar one. Astronomers have called this intercalary lunation as adhika month, and have asserted that this month is to be added to the lunar year.
Adhika (Purushottam) Maase Since the calendar is based on the phases of the Moon, the transits take 354 days, 8 hours and 34.28 seconds. This creates a difference of 10 days, 21 hours and 35.16 seconds from the actual solar year (365 days, 6 hours, 9.54 seconds) When the accumulated difference exceeds 29 days, 12 hours, 44 minutes and 2.865 seconds, an adjustment is made with an extra month (Adhika Maas), which carries the name of the previous or the next month, depending on the proximity of the month. Normally, seven extra months occur in 19 years.
Astronomical & Astrological technicalities explained regarding origin of, and purpose of Adhika mase, and also symbolically what the Adhika represents.
Purushottama adhika masa vratas from Mahanidhi Swami, submitted by Mahamantra (das) ACBSP (Vrindavan – IN) 
…this may be useful to you or yours Recommended Vows for Purusottama Month
(2001 18 September-16 October) The main vrata is to worship Radha Krishna Yugala
***Rise Brahma-muhurta DAILY (bm means one . hours before sunrise til one
hour; if sunrise at 6am then bm starts 4:30 goes to about 5 a.m.) Means
taking rest EARLY night before.
***take vow to REMAIN TRUTHFUL for whole month.
***Careful not to BLASPHEME any devotee,sastra, brahmin, cows, saints, or one observing the Purusottama Vrata.
1.fixed increased chanting 25, 32, 64,128 rounds daily!
2. hearing hari katha (Study Krishna Book (chapters dealing with Vraja lila and Krishna meeting gopis at kuruksetra) Srimad Bhagavatam & Chaitanya Charitamrita specifically desc. Radha Krishna guna and lila) and/or speaking daily hari-katha
3. worshiping Radha Krishna, which means, unless you have deity with = you, then daily offer fresh first class lotus or rose plus tulasi = garlands to Radhe Shyam photo or painting of Radha-Govinda Yugal.
*if worship Salagrama (could apply to Radha-Govinda also) offer 100,000 tulasi leaves and achieve “unlimited merit beyond the powers of Brahma to describe.”
4. Sing bhajans and kirtans specifically glorifying Yugala Kishora. Such as Jaya Radha Madhava; Radhe jaya jaya madhava dayite; radha-krishna prana mora; Radha-krpa kataksa (in back our radhakunda book or in Giti Guccha song book; also sing Nandanandana Astakam (also in Giti Guccha song book); jaya radha madana mohan.. sanatana prana dhana he; jaya madhava madana murari radhe shyam shyama shyam; radha-krsna bol bol bolo re sobhai; vrndavana ramya sthana; tulasi arotik prayers with deep prayer of longing and meditation;
5. MUST OFFER Daily GHEE (pure cow’s ghee only) lamp to Radha-Krishna Deity or photo.
6. Bathe DAILY in Yamuna around sunrise. Or if not in holy land then pour drops on head from bottle of Radha-kunda water or Yamuna after offering obeisances and prayers to Radha-kunda or Yamuna.
7.Eating–Fast till noon and eat only fruits or just vegetables without any beans or grains. (like ekadasi preps for one month!) once a day if you can. Or just eat mahaprasadam, full to neck ONCE A DAY. Take a vow according to your capacity to follow, and in no condition can one break his vow in the middle of the month.
HOW TO BREAK THE PURUSOTTAMA VRATA AFTER ONE MONTH?
1. Worship Radha-Govinda and chant this mantra: “O Supreme Lord! O Sanatana! O Purusottama! O Lord Hari! I offer my obeisances unto You. May You and Your beloved Srimati Radhika please accept my offerings. I offer my repeated obeisances unto Radha-Syamasundara, who wears effulgent yellow garments on His beautiful body.”
2. Offer pushpanjali and dandavats to Radhe-Syama.
3. Give charity to 3,5,7,9, 11 brahmanas to best of your ability.
**Best charity to give Srimad Bhagavatam to qualified Vaisnava devotee brahmana. This charity will deliver all you forefathers to Goloka dhama and the association of Lord Purusottam!
***Other recommended gifts: new clothes, gold (rupees, bucks, pounds), two pairs of new shoes
4. Feed brahmanas with prasadam
SPECIAL ACTIONS AT END OF MONTH:
1. If not bathed in sacred waters—nullify reaction by donating milk and yogurt to brahmanas
2. If eat rice, wheat, fruits,oil, or ghee during P.M.-give charity to brahmanas (as mentioned above)
3. If eat on leaf plates for month-then feed brahmanas with ghee and sugar
4. Not cut hair and nails for P.M.-give mirror to one brahmana.
IF BROKE ANY RULES DURING PURUSOTTAMA MONTH:
Must feed brahmanas with different kinds of sweet juice. (sugar cane, oj, grape, etc)
BENEFITS OF OBSERVING PURUSOTTAMA VRATA
1. All the holy places in the world come to reside in your body.
2. One who observes Purusottama Month with faith and devotion will go back home, BTG to serve Radha-Govinda eternally in Goloka Vrndavana at the end of his life!
These are the recommended vratas described in Padma Purana PUrusottama = Mahatyam. We are sure Prabhupada will help you honor this month nicely.
Note: Special thanks to Jaya Tirtha Caran prabhu from NZ for allowing us to use some of the content from his site to compile these pages.