In the Northern Hemisphere, on which date does the longest day of the year normally occur due to the summer solstice?

Introduction / Context:
The apparent movement of the Sun across the sky during the year is caused by the tilt of Earth rotational axis and its revolution around the Sun. As a result, the length of day and night changes with seasons. The longest day in the Northern Hemisphere occurs at the time of the summer solstice, which is an important concept in geography and general science. Many exams test whether you know the approximate date of this event.

Given Data / Assumptions:

- The question is restricted to the Northern Hemisphere.
- It asks for the date of the longest day of the year.
- Options provide four specific dates in different months.
- We assume a typical year and usual calendar reference where the summer solstice occurs on a nearly fixed date.

Concept / Approach:
Earth axis is tilted about 23.5 degrees relative to the plane of its orbit. Because of this tilt, different hemispheres receive varying amounts of sunlight during the year. When the North Pole is tilted most toward the Sun, the Northern Hemisphere experiences maximum daylight duration, which is called the summer solstice. This occurs around 21 June each year. On this date, locations in the Northern Hemisphere enjoy the longest daytime and shortest night of the year.

Step-by-Step Solution:
Step 1: Recall that solstices and equinoxes mark special positions in Earth orbit and the tilt of the axis.Step 2: The summer solstice in the Northern Hemisphere occurs when the Sun apparent path is farthest north in the sky.Step 3: Textbooks and almanacs usually state that this solstice falls on or around 21 June.Step 4: On this date, the Northern Hemisphere experiences its longest day and shortest night.Step 5: Therefore, the correct option for the longest day in the Northern Hemisphere is 21st June.

Verification / Alternative check:
A quick mental check is to remember the general pattern: around 21 March is the spring equinox, around 21 June is the northern summer solstice, around 23 September is the autumn equinox and around 22 December is the northern winter solstice. Among the given options, only 21st June fits the well known date of the northern summer solstice. Recognising this standard sequence makes it easier to answer questions about seasons and day length.

Why Other Options Are Wrong:
20th May: This is about a month before the usual summer solstice and the day length is still increasing but has not yet reached its maximum in the Northern Hemisphere.
20th July: By this time the solstice has passed and day length is already gradually decreasing again, so it is not the longest day.
21st August: This date falls well after mid summer, and the length of daylight is clearly shorter than in June, so it cannot be the longest day.

Common Pitfalls:
Students sometimes confuse solstice dates with equinox dates, or mix up which solstice corresponds to which season in the Northern Hemisphere. Another common error is assuming that the hottest days of the year coincide exactly with the solstice, even though temperature peaks often appear later due to seasonal lag. Focusing on the definition of solstice as the time of maximum or minimum daylight rather than maximum heat helps avoid this confusion.

Final Answer:
In the Northern Hemisphere, the longest day of the year normally occurs on 21st June.