Questions on script and "sacking"

[LorDBulA]

I_TurnNumber = 21 is what you want.
This peace of script:

monitor_event FactionTurnStart FactionType greek_cities
and I_TurnNumber = TURN_NUMBER

will ensure that inside of this monitor will be executed only on turn equal to TURN_NUMBER and only at the begining of greek turns.
It will be executed only once.