I'm not sure I fully understand the question but during a total solar eclipse, the moon's shadow covers a small strip on the Earth, not the sun. The sun is the source of light. I'm guessing this is what you are asking.
There are several special things about the relationship between the Earth, the sun, and the moon. First, the sun is about 400 times bigger than the moon but is also 40 times farther away from the Earth than the moon making the two appear about the same size in the sky. The moon's orbit around the Earth is not on the same plane as the sun but instead is 5 degrees off. This results in the moon lining up with the Earth and the Sun only 4 to 7 times a year at what are called nodes. If it is a new moon then a solar eclipse will happen. If it is a full moon then a lunar eclipse will happen. Solar eclipses can be total when the moon is closer to the Earth and completely blocks the sun. If the eclipse happens when the moon is furthest from the Earth there will be an annular eclipse where the sun appears as a ring as it will look to be slightly bigger than the moon.
As for the day before the eclipse, it takes 28 days for the moon to orbit around the Earth. So the day before a solar eclipse the moon is a day away from casting a shadow on the Earth. Maybe the easiest way to think about it is the moon is like the hour hand on a 28-hour clock. It can only line up with the sun when it is noon, so when it is say 27 o'clock it is nowhere close to casting a shadow on the Earth. The flip side is, that only when the moon is full at say 14 o'clock can there be a lunar eclipse where the Earth blocks the sunlight from the moon. Add to the complexity of this is the 5-degree wobble the moon has with the plane the Earth is on. So not all new moons will lead to an eclipse. Occasionally the planes will be close enough that only a partial eclipse can happen. For a total eclipse all three bodies have to be lined up and either the moon has to be closer to the Earth at the time or the Earth has to be at its furthest point from the sun. Remember both the moon and Earth do not have a circular orbit but rather an elliptical one.
I hope I answered your question. The link I have provided is excellent and worth a look if you have further questions.
https://coccweb.cocc.edu/bemerson/PhysicsGlobal/Courses/GS107/GS107Materials/GS107Breadcrumb/GS107BCSunEarthMoonII.html
