PURANMAASHI
Literally: Full (day) of month, i.e. the night of the full moon. It is opposite of Amavas/ Massiya (the night of no moon).
Among the Hindus Sangrad, Amavas, Pooranmaashi, Pnachmi, Ekadashi and many more days (and dates and hours) are either auspicious or inauspicious.
In Sikhism there is no auspicious or inauspicious day or hour hence Puranmaashi, Massiya, Sangraand etc. are of no importance for a Sikh.
The Sikhs celebrate the birthday of Guru Nanak Sahib on the Puranmashi of Kattak month of the Bikrami calendar (which fell on 20th October in 1469), not because of Pooranmaashi but because it was co-incidence that the Guru was born on that day.
The Nirmala-Udaasi preist who had remained in occupation of the Gurdwaras up to 1925, had begun celbrating Puranmaashi, Massiya and Sangraand in Darbar Sahib and other Gurdwaras. When the Sikhs took control of the Gurdwaras from these priests, celebration of these days was stopped. In 1962 Fateh Singh captured the Shiromani Gurdwara Parbandhak Committee and he started again celebration of these days. This is against the teachigns of Guru Granth Sahib.
(Dr Harjinder Singh Dilgeer)