As for that night, let darkness seize upon it; let it not be joined unto the days of the year, let it not come into the number of the months.
Seize upon it - as its prey; i:e., utterly dissolve it.
Joined onto the days of the year - rather, by poetic personification, 'Let it not rejoice [yichal] in the circle of days and nights, and months, which form the circle of years.'