Listing days of the week in a ketubah

Hi,

When we put dates in a legal document, like a ketubah, we spell the day of the week using the numbers listed in Bereishit, and the spelling used there. For example, we write חמישי with two yuds.

In Bereishit, the first day of the week is called יום אחד, Day One, rather than יום ראשון, First Day. This is odd, given that the rest of the numbers are ordinal [שני, שלישי, רביעי, second, third, fourth] rather than cardinal [שנים, שלש, ארבע, two, three, four].

For a legal document we also use the cardinal number of יום אחד, because that's the Torah's language.

(Nachalat Shivah 5)

Have a great day,
Mordechai