[dredmorbius]

The demand can be generally predicted for at least several days in advance based on weather. This affects both generating capacity (incident solar, wind) and loads (hours of daylight, heating or cooling load). The process isn't perefect, but converges on experience the closer to present you are. Factors such as predictable human activities (workday, workweek, and seasonal factors) also enter in.

The events you may have noted in news of "negative energy prices" are often failures of prediction -- unexpectedly high availability (more sun or wind), and unexpectedly low demand. Though "pay to take my power" sounds good, it's actually a sign of mismanaged resources.

There are occasional incidental factors -- sudden demand, or more often, equipment or transmission failures which require bringing additional capacity online, or shedding load to prevent under-voltage (and hence: over-amperage), or underfrequency. Grid power frequency is generally 60Hz in the US, 50Hz in the UK, and just for grins, both in Japan, on separate and noninterdependent grids, which made generation capacity loss following the Tohoku earthquake/tsunami and Fukushima incident all the more critical. Loss of synchronisation or deviation by more than a very small fraction from the nominal frequency is considered a Very Bad Thing. Viz the recent UK blackouts.