Gajachchhaya Yoga is formed by combination of Sun, Moon and 2 tithis (Lunar Days) which are Trayodasi and Amavasya).
This yoga is formed when sun transits through Hasta Nakshatra in Virgo (Kanya Rasi) once in a year and mostly during Pitru Paksha.
The combination when the Sun is in Hasta Nakshatra and the Moon is in Magha Nakshatra during Trayodasi Tithi of Pitru Paksha is known as Gajachchhaya Yoga.
Next occurance of Gajachchhaya Yoga is when the Sun and Moon are in Hasta Nakshatra during Amavasya Tithi of Pitru Paksha is also known as Gajachchhaya Yoga.
So, there might be one or two Gajachchhaya Yoga during Pitru Paksha. This auspicious combination occurs either on Trayodashi Tithi or on Amavasya Tithi.
Gajachchhaya Yoga (गजच्छाया योग) does not form every year. But whenever Gajachchhaya Yoga occurs during Pitru Paksha, it is considered highly auspicious for performing Shraddha rituals and charity.
Transit of Sun in Hasta nakshatra happens every year during September-October. But this should be Bhadrapada Maasa (where fullmoon forms in Uttara Bhadrapada Nakshatra) and not adhika maasa.
Second half of this month (when Moon is waning between fullmoon to next newmoon) is called Mahalaya Paksha or Pitru Paksha and no auspiscious event is performed during these 15 days.
Traditional astrologers do not check any horoscopes and postpone all astrological advises.
These are the days to offer Shraddh to deceased ancestors in a family.
Pitru Dosha Causes, Effects & Remedies.
Curses of forefathers is said to cause pitra dosham according to puranas, but reality is that, one faces this only because of his/her past life deeds.
There are be millions of souls in this world who are not resting in peace. All the souls are ancestors of someone who are alive today.
But, reality is that Pitra devatas are not the one who are deceased, but the ones who are never born and will never die.
They reside in the form of DNA and continue that ‘Gotra‘ through ‘Y’ chromosome of males in that family.
When one person suffers from pitru dosha, his/her family will struggle to see next generation smoothly.
Either they will not have kids, or their children will seperate from them, or no male kids in next generation will end their Gotra.
Trayodashi Gajachchhaya Yoga Starts on 6th October 2018 from 16:42 hrs IST
Trayodashi Gajachchhaya Yoga Ends on 6th October 2018 at 17:01 hrs IST
Duration of Gajachchhaya Yoga on Trayodasi is 0 Hours 19 Minutes.
Amavasya Gajachchhaya Yoga Starts on 8th October 2018 from 13:24 hrs IST
Amavasya Gajachchhaya Yoga Ends on 9th October 2018 at 09:17 hrs IST
Duration of Gajachchhaya Yoga on Amavasya is 19 Hours 53 Minutes
Gajachchhaya Yoga is useful when it falls during day time and it has no value when it falls during night time.
So, in 2018 Pitru Tarpan or Shraddh should be performed by people whose father passed away on 3 occassions (evenings of 6th, 8th october and morning of 9th october 2018).