Launch happens when trajectory plane contains launch site, launch asymptote and center of Earth. On a launch day there will be two launch opportunities, one at morning and other at the night. Which launch parameter makes these two launch time separate?
There will be roughly two opportunities per day if
- The launch site's latitude is less than the orbital inclination,
- The launch is directly into the desired orbit, with orbit insertion happening when the launch vehicle reaches the desired altitude (i.e., no transfer orbit), and
- All you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit.
The orbital plane is roughly fixed in inertial space. (Note: This ignores precession due to the non-spherical nature of the Earth and due to external bodies. That's why I said "roughly.") The Earth's slightly less than 24 hour rotation rate (the sidereal day) means that the launch site will switch from being on one side of the orbital plane to the other side twice every sidereal day. Add in precession and this will change somewhat, but it's still roughly twice per day.
That last condition, all you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit, usually does not apply. Space vehicle operators tend to care a lot about where the vehicle will be on the orbital plane. For example, it's rather important if the vehicle is to rendezvous with some other object, if it is to be in a particular sun-synchronous orbit with a particular local sub-satellite point time, or if it is to be in just the right spot at geosynchronous altitude.
TL;DR version: Which parameter causes two launch times in a launch day? The answer is the Earth's ~24 hour rotation rate.