«فِي لَيْلَةٍ فَتِيَّةِ الشَّبَابِ»
On a night portrayed as fresh with youth.
Fatiyyah suggests youthful freshness; shabāb means youth. Night is described as if it could pass through a human stage of life, producing personification and metaphor.
The expression conveys the full freshness of the night and the travel time ahead. It is a literary description of time, not a literal statement that night is a young person.