In the northern hemisphere, the longest day of the year normally occurs in the:

The longest day of the year in the northern hemisphere, also known as the summer solstice, typically occurs in the second half of the month of June.

This is when the sun is directly over the Tropic of Cancer, or more specifically right over 23.5-degree north latitude.

The exact date can vary slightly from year to year, but it is usually on June 20th or 21st.