What do Pradoṣa, Saṅkaṣṭī, Pūrṇimā and Amāvasyā mean for practice?

they are recurring monthly holy days — for Śiva, Gaṇeśa, Viṣṇu/Satyanārāyaṇa and the ancestors respectively.

Pradoṣa falls on the trayodaśī (13th tithi) of each fortnight. Tradition says Śiva dances in joy at the twilight hour, so devotees fast during the day and worship Śiva at sunset with abhiṣeka, bilva leaves and a lamp. When it falls on a Monday (Soma Pradoṣa) or Saturday (Śani Pradoṣa) it is considered especially auspicious.

Saṅkaṣṭī (Saṅkaṣṭa Caturthī) is the 4th tithi of the dark half, for Gaṇeśa, the remover of obstacles. People fast and break the fast after seeing the moon rise and offering it water. Pūrṇimā, the full moon, is loved for the Satyanārāyaṇa Kathā, temple visits and charity; Guru Pūrṇimā and Kārtika Pūrṇimā are high points. Amāvasyā, the new moon, is a quiet day for remembering ancestors with tarpaṇa and for inner practice.

You do not need to keep all of these. Pick one that speaks to you and keep it regularly; steadiness matters more than number.

Source: Skanda Purāṇa; common practice · Pradoṣa = 13th tithi at twilight

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