How did the word for “day” change from Proto-Germanic into Old Norse and Old English?

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The Proto-Germanic word for "day" was *dagaz. As the Germanic languages evolved, this term underwent specific phonological changes in different dialects:1

Old Norse: The word became dagr. In North Germanic, the final Proto-Germanic z was maintained and appears as r*.1
Old English: The word became dæg. In West Germanic (which includes North Sea Germanic), the final *z was generally lost in monosyllabic words.1

These developments illustrate the broader linguistic shifts that occurred as the Proto-Germanic language fragmented into distinct regional branches.1