Can I fast only on the first and last day of Navarātri?

yes; fasting on Pratipadā and Aṣṭamī/Navamī (the “joḍā” or “pahlā-ākhirī”) is a long-accepted custom.

Not everyone can fast for nine days, and the tradition has always made room for that. Common patterns include: fasting all nine days; fasting on the first and the last day only; fasting on Saptamī, Aṣṭamī and Navamī (the trirātra, especially in the East); or simply eating once a day without grain throughout. Each is a complete Navarātri vrata if it is your saṅkalpa.

The key is to decide on the first morning what you will keep, say it in your saṅkalpa, and then keep it. On non-fasting days, many still avoid onion, garlic, meat and alcohol in the home, and keep the evening āratī.

Students during exams, people doing hard physical work, nursing mothers and anyone on medication should choose the lightest form or simply offer prayer. The Goddess is Mother; she does not ask her children to harm themselves.

Decide your form in the first-day saṅkalpa · Source: common family practice

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