ऋतोरण्

Operational rule vidhi

The affix अण् comes in the sense of 'that has come for it' after the word ऋतु.

Word by word (padaccheda)

Left context ऋतोः pañcamī · ekavacanaSubstitute / term अ prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

ऋतोः अण्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix अण् comes in the sense of 'that has come for it' after the word ऋतु।
As ऋतुःप्राप्तोऽस्य = आर्त्तवं पुष्पम् (Guna by 6.4.146) "the flower for which the season has arrived".

Vart:- So also after the words उपवस्त्र &c. As औपवस्त्रम्, प्राशित्रम् ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.