Why and how much does the weather get warmer after April 15th in Norther Hemisphere?

There are more daylight hours, and the sun's rays hit the Earth more directly,instead of at a slant. The date is not fixed. It's just that by April it becomes more noticeable, Average temps get gradually warmer after January. Actual temps also depend on air mes such as storms and cold fronts ,and their movements, so you can have freezing spells late in spring, or warm spells in January at times.