For the medievalists in the house…

I’m puzzled by some lines (ll. 150-152) in Pope XIa on Easter.

And we wurðiað þa tid wurðlice mid sangum
Seofon niht on an, swylce hit an dæg sy,
For ðære micclan mærðe manncynes alysednysse.

Literally:
And we celebrate the time worthily with songs
[?seven nights in one, as if it were one day?]
for the great joy of humanity’s redemption.

What’s up with line 151? The combination of the prep ‘on’ and the swylce+subj. leaves me unsettled (my grammatical skills in OE still suck…). As a liturgist I immediately think that he’s talking about the extraordinary length of the Easter Vigil but, not having neither the RC or the LME close to hand, don’t remember how long their Vigil was…

Thoughts?