In Jyotish, for Panchak Yoga, ‘panchak’- group of five… refers to the five lunar constellations, i.e., nakshatras which are Dhanishta (last two charanas or padas), Shatabhisha (also called Shatataraka), Purva Bhadrapada, Uttara Bhadrapada and Revati. When moon transits these nakshatras, it is considered as a highly inauspicious time to begin any new activities or conduct any special events such as marriage, mundan (thread ceremony) and so on.
You can note which nakshatra moon is traversing on any given day, by referring to your local panchang. Since the transit is dependent on the geographical location at any given time, it is important to consider a panchang that has specifically been calculated for your geographical location.
Nakshatras like Revati and Dhanishta which are very auspicious otherwise in terms of worldly gains… why do we not perform auspicious activities when moon is traversing them? Though there are no authoritative shastras that explain this scenario, we can gain some insight into it from the basics of Jyotish (Vedic Astrology) itself!
When we consider the Kaal-Purush kundali, which is arrived at by considering Aries as the ascendant and indicates the natural positioning of the various houses. For the Kaal-Purush kundali, we find that the zodiac signs of Aquarius and Pieces fall in the 11th and 12th house respectively.
It is believed that the astrological houses 1 thru 10 are meant for this material world and that the houses 11th and 12th indicate our spiritual journey. That is the reason why we think of 11th house as the gains that we acquire from our work in this world (1 thru 10th house). 11th house also indicates our journey into the cosmic waters of Pieces in the 12th house.
Now, moon indicates the progression of our Mann (loosely translated in English as the mind… though from the Yogic perspective, it encompasses much more than that) … Moon is also the storehouse of our Causal Body as indicated in Ayurveda and hence is also the dispenser of our karmas! If you have not read my article about our 'many bodies' then, so check out: Our Numerous "Bodies" (Understanding Karma & Karmic Clearing series)
When moon in transiting the last two spiritual houses of Aquarius and Pieces, would indicate a time when we should withdraw our energies from the material pursuits and instead focus on our spiritual self. This would be the time when we should focus on our spiritual progress and wellbeing. Instead if one is to continue with their material activities and pursuits, then it is but natural that they will not attain success, as the time would not be conducive for such mundane, material activities.
That would also explain why it is only the last two padas of Dhanishta nakshatra which is included in the Panchak Yoga, since the first two padas of Dhanishta fall in the zodiac sign of Capricorn and would be in the 10th house of the Kaal Purush Kundali.
Our Rishis were very scientific in their approach. Time was truly understood and best utilized to help us lead a successful life… materially as well as spiritually. That is the reason, our Rishis explained to us about time periods like the Abhijeet Kaal where we can take up material activities for positive, beneficial results and then there are also phases like the Panchak Kaal when we are better off refraining from worldly activities and instead should focus on our spiritual practice.