Most events are triggered based off a combination of girl stats, location and time. And some have other conditions. But basically you have to be in the right place at the right time and usually have the correct stats for things to trigger.
Like say you have to be driving home at night during a storm or be in the ballroom between 12:00 and 12:20 and have a certain girl etc. Both of those are real event triggers by the way. This basically means that the game unlocks it self as you slowly explore and level. But it takes a lot of grinding to get there and as mentioned in my examples the triggers are not always obvious.
That's a cherry picked item, but you are correct that some scenes are harder to trigger than others.
In the case of Sidra's prayer times, it's a fairly small time window. The 20 minutes you mention, is three times a day, every day.
In the case of meal times, it's a one hour window three times a day.
The vast majority of the game's scenes are grouped by time range:
Work hours: 6am-6pm
Waking hours: 5am-10pm
Non-Work Waking hours: 5am hour, 6pm-10pm
Night: 10pm-5am