Could be lots of things.
Try this. When it won't start, floor the gas pedal and keep it floored. If the car starts up (perhaps a little reluctantly) then it's been flooded with gas while it sat, perhaps by a leaking injector. Naturally, after the car starts up, you let your foot off the gas. Don't race the engine.
Other than that, if that doesn't work, you might have a mechanic test the fuel pressure in the fuel rail at the time it won't start. Perhaps your fuel pressure is bleeding off during the day and this contributes to the hard start.
If that doesn't work, have the mechanic test for spark during the non-start. Maybe a heat-sensitive ignition module or a bad crankshaft positioning sensor?
Sounds like everyone's been guessing and nobody's measuring anything.