The affix aṇ comes in the sense of 'that has come for it' after the word ṛtu,
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 ॥,