First time Julius Caesar appeared on world's stage, for instance - a window of opportunity and a dark period? (7,5)

'first time julius caesar appeared on world's stage for instance a window' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'opportunity and a dark period?' is the wordplay.
'opportunity' becomes 'opening' (opening is a kind of opportunity).
'and' says to put letters next to each other.
'a dark period?' becomes 'night' (I've seen this before).
'opening'+'night'='OPENING NIGHT'

'of' is the link.

Can you help me to learn more?