والبيع إلى النيروز والمهرجان وصوم النصارى وفطر اليهود - إذا لم يعرف المتبايعان ذلك فالبيع فاسد
Translation: Where payment is deferred to occasions such as Nowruz, Mihragan, Christian fasting, or a Jewish feast, the sale is defective if both parties do not know the relevant date.
The problem is not that a holiday name is itself a forbidden unit of time; it is that the payment deadline is unknown to the parties.
If both parties know the calendar date clearly, this particular uncertainty is removed.
Examples:
1. 'Payment on a named festival' when its date is unknown: dispute is likely.
2. 'Payment on November 15': a definite date.
The holiday names function as historical calendrical references.