पक्षयोरुपवासं च चतुर्दश्यष्टमीषु च
pakṣayorupavāsaṃ ca caturdaśyaṣṭamīṣu ca
One should also observe fasting (upavāsa) in both fortnights of the lunar month, especially on the fourteenth day (caturdaśī) and the eighth (aṣṭamī)—disciplines that purify the paśu and steady it toward Pati, Lord Śiva.
Suta Goswami (narrating Shiva-dharma and vrata-vidhi to the sages of Naimisharanya)
It prescribes upavāsa (fasting) on sacred tithis—especially caturdaśī and aṣṭamī—as a core preparatory discipline that supports purity, restraint, and focused devotion before Linga-pūjā.
By directing the practitioner toward regular austerity, it implies Śiva as Pati—the transcendent Lord approached through inner purification, where the paśu becomes fit to receive grace (anugraha) by loosening pāśa (bondage).
Upavāsa as vrata/niyama: a Shaiva observance aligned with Pāśupata discipline, using lunar tithis (aṣṭamī, caturdaśī) to stabilize mind and senses for worship and japa.