दिवसश्च पृथिव्याम्

Operational rule vidhi

For दिव् is also substituted दिवस् (as well as a द्यावा) when पृथिवी follows in a द्वन्द समास of names of देवता-s.

Word by word (padaccheda)

Substitute / term दिवसः prathamā · ekavacanaIndeclinable च avyayaRight context पृथिव्याम् saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

Full reading

दिवसः च पृथिव्याम्

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

Commentary

For दिव् is also substituted दिवस् (as well as a द्यावा) when पृथिवी follows in a द्वन्द समास of names of देवता-s.
Thus दिवस्पृथिव्यौ, द्यावापृथिव्यौ ॥ The word दिवस् is exhibited in the aphorism as दिवस् ending in अ, in order to indicate that the स् of दिवस् is not changed to ह् or a visarga or upadhmaniya when followed by पृथिवी ॥ Following is an anomalous use there being no Dvandva compounding here. द्यावा चिदस्मै पृथिवी ममेते ॥

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