I want to start keeping Ekādaśī — which one do I begin with, and what about the day before?

many begin with Utpannā Ekādaśī, but any Ekādaśī is fine; on Daśamī eat a light single meal without grain at night.

Families who start formally often choose Utpannā Ekādaśī (Mārgaśīrṣa kṛṣṇa), since it marks the birth of the Ekādaśī devī; others begin with a śukla pakṣa Ekādaśī or simply the next one coming. Avoid starting in Cāturmāsa or Adhika māsa only if your family has that custom.

Ekādaśī really spans three days. On Daśamī, eat one sāttvic meal, ideally before evening, and skip grain at night so you wake up light. On Ekādaśī morning make a saṅkalpa before Viṣṇu or Kṛṣṇa, and fast as your health allows. On Dvādaśī, break the fast within the pāraṇa time and, if possible, feed someone or give a small dāna.

Start with a fruit-and-milk fast; move to stricter forms only if they suit your body.

Three days: Daśamī – Ekādaśī – Dvādaśī · Source: Padma Purāṇa (Utpannā Ekādaśī māhātmya)

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