I have checked several Arabic grammar books (Thatcher, Tritton, Faruk Abu-Chahra, Abdul Sattar & Cowan) and apart from Cowan, they all give the name of this month as: ربیعُ الاولُ. Cowan, however has this as rabii3uni_l2avvalu.Thank you for your input, Hemza! I have looked into this some more. The preferred forms do indeed seem to be rabīʻu l-ʼawwali and rabīʻu l-ʼāxiri, which are analysed as possessive constructions of the type styled ʼiḍāfatu t-tafsīr or ʼiḍāfatu l-bayān (Wright, Arab. grammar II pp. 232-3; Reckendorf, Arab. Syntax p. 140.), as in the Qur’anic dāru l-ʼāxirati (16:30), if this in fact means the same as ad-dāru l-ʼāxiratu ‘the final abode’. Still, it seems very strange that rabīʻu l-l-ʼāxiri and jumādā l-ʼāxiratu should be construed differently.