General:
* Daylight
* Daytime
* Daybreak
* Dawn
* Dusk
* Sunset
Time:
* 24 hours
* Period
* Epoch
* Era
Specific types:
* Weekday
* Weekend
* Holiday
* Working day
* School day
* Summer day
* Winter day
Figurative:
* Golden age
* Dark ages
* Prime time
* Moment in time
Related to the sun:
* Sunrise
* Solar day
Related to the moon:
* Lunar day
Abstract:
* Opportunity
* Chance
* Hope
The best noun to use will depend on the specific context you're writing in.