Is it valid to stipulate a term for a Salam contract based on non-lunar calendar measurements known among Muslims?
Chapter on Guaranteed Salaf (Advance Payment) for a Specified Term
Al-Mughni
Book of Sales
Primary text
If the term is specified by a measure not based on lunar months but is known and famous among Muslims, such as the months of Kanun or Shubat (winter months), or fixed festivals like Nowruz or Mahrajan (for those who recognize them), the apparent view of Al-Kharaqi and Ibn Abi Musa suggests it is invalid because the term is stipulated using non-lunar months. This is likened to stipulating delivery at Sha'banin or the festival of the scattering of bread (Eid al-Fitir) because many Muslims do not know these non-lunar terms.
Supporting text
Al-Qadi, Al-Awza'i, and Al-Shafi'i hold that it is valid if the term is known and does not vary, similar to established Muslim festivals. Al-Awza'i stated that stipulating delivery until the Christian Easter or their fast is permissible because it is known and fixed. They differentiate this from terms that vary, which Muslims are unaware of.