We do not have precise dating for archaic day names like Hihdei, except for a comment that they were in use 'at least nine hundred years' before the end of the Third Age (according to Appendix D to The Lord of the Rings). They were therefore in use in about III 2100, and at some point between that time and the end of the Third Age names like Hihdei evolved into the more recognisable Highday and its equivalents.

Our only specific record of this usage is from the history of the Shire. Its use in Buckland or the Bree-land seems likely, but is not directly attested. For further discussion on this point, see footnote 2 to the entry for Sterrendei.

Lacking a direct explanation of how to pronounce the word Hihdei, our best approximation would be to follow Old English (the language from which the word originates). On that basis, both the h sounds would be pronounced as something close to Scots ch in the word loch, and the i sound would be short, as in 'him' rather than modern 'high'. The final -dei incorporates the diphthong ei, pronounced as 'ay', so the dei element is pronounced like modern English 'day'.