This is a great question! Solar scientist can not predict when a solar flare will occur! We only know that solar flares occur in large, complex sunspots where the magnetic field lines are tangled up so that lots of energy is stored in the magnetic fields. Trying to predict solar flares is an important area of research. Solar flares can be dangerous to rockets, spacecraft and astronauts because large solar flares are accompanied by large fluxes of "solar energetic particles" which can cause damage to both human and robotic space explorers.