General:
* Situation
* Condition
* State
* Event
* Occasion
* Factor
* Aspect
* Element
* Detail
More specific:
* For events: Incident, happening, occurrence
* For challenges: Difficulty, obstacle, hardship, predicament
* For surroundings: Environment, atmosphere, setting, context
* For luck: Fortune, fate, destiny, chance
More formal:
* Contingent
* Conjuncture
* Predicament
Figurative:
* Turn of events
* Twist of fate
* Stroke of luck
The best synonym will depend on the specific meaning you want to convey.