In his book on Minhag in early Ashkenaz, I. M. Ta-Shema hid this in a footnote; I thought it would be of interest to the community.

It seems to me that the source of the [Ashkenazi] custom to separate the [marriage] cermony [and hold the betrothal ceremony on Friday, and the huppah on Saturday morning] is the early custom, which was apparently defunct by the 11th century, to sign the ketubah only after the marriage was consummated (בעילת מצווה). And see the words of the Italian payytan Amitai b. Shefatiah, in the 9th century:

ומה נאה לחתן ביום חתונתו

להראות טהרת בתולי אשתו

בראש תלוי לחתום בכתובתו

ולעשות שלימה שמחתו

עד יום צאת טבילה

And how fitting for a bridegroom on the day of his wedding

to show the very purity of his wife’s virginity

with his head held high (?) to sign his ketubah

and to make his happiness whole [i.e. to have sex with his wife]

until the day the immersion is due

(I. David, The Poems of Amitai, Jerusalem 1975, p. 23, ll. 227-229; my hasty translation)

And from this we learn two things:

(1) the ketubah was attested and became valid only after consummation; (2) at this time the ketubah was signed in public and was accompanied by a celebration, and was one of the high points of the marriage ceremonies […] At the same time the blessing “who planted a nut” was said, see Halakhot Gedolot, ed. Hildesheimer, part II, Jerusalem 1980, p. 226.

I. M. Ta-Shema, Minhag Ashkenaz ha-Kadmon, p. 43 n. 50.

Edited 12 September @ 15:09; “mitzvah penetration” was changed to “consummation,” due to popular demand.