A dvanda compound of words denoting kratu (sacrifices) ordained by yajur (adhvaryu) Veda, is singular, provided they are never used in the neuter gender,
As, अर्काश्वमेधम् the Arks and Asvamedha sacrifices,' सायाह्नातिरात्रम् 'the Sayahna and Atiratra sacrifices.
Why do we say 'Yajur-veda sacrifices' ? Observe इषुवज्रौ 'the lshu and Vajra ceremonies,' उद्भिद्वलभिदौ ॥
Why do we say 'when not in the neuter gender' ? Observe राजसूयवाजपेयौ 'the Rajasuya and Vajapeya sacrifices.'
Why have we दर्शपौर्णमासौ ? Because the word क्रतुः is a technical term applying only to Soma-sacrifices and not any sacrifices in general. Therefore, though Darsa and Paurnamasa are Yajur-veda ceremonies, they not being Soma ceremonies, the present rule, does not apply.,