why does the star twinkle at night and not during the day?

Starlight travels through the many layers of the Earth 's atmosphere before it reaches to our eye. Light coming from star gets refracted many times in random directions in atmosphere. Thus image of star appears somewhat displaced from its actual position. Since, atmosphere has both cold and hot air and by changing width of these layers of air light ray bends less or more. So, image of star appears changing its position. This is twinkling of star.

Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so is subject to more refraction.

Stars would not appear to twinkle if we viewed them from outer space.

During day time the sky is so bright beacuse of scattering of sun light. Therefore stars can not be see twikling during day time.