The longest day and the shortest night of the year in the Northern Hemisphere occur during summer solstice which is usually observed on june 21st or sometimes on june 20 UTC (see table 1). In 2020 summer solstice is observed on june 20, exactly at 21:44 UTC. The opposite case takes place during december solstice - the shortest day and the longest night occur in the Northern Hemisphere and the longest day and the shortest night are observed in the Southern Hemisphere. The day of june solstice is also the first day of summer in Northern Hemisphere and the first day of winter in Southern Hemisphere.As you can see on Figure 1,2 and Table 2,3, the length of the day depends on the latitude of the place on the Earth - the higher the latitude, the longer the day but the dependency is nonlinear (see Figure 2). Table 3 shows some cities located on different latitudes around the World sorted by latitude in descending order, as you can see, the day is longer, for example, in New York City, NY (latitude: 40° 45' 10'' N) then in Los Angeles, CA (latitude: 34° 03' 10'' N). Extreme situation takes place far on the North (and South) where the sunrise and the sunset do not occur - there is no night in the first case (north) and no daytime in the second (south). So, answering the question '', the longest day on the Earth lasts 24 hours, but more accurately, you could say - ''.Exact day length, sunrise and sunset time for any day of the year for over 200.000 places can be found here: Sunrise & sunset database , for example: Longest Day of the Year in London, UK As shown in table 2 the daylight duration on the Equator is not exactly 12 hours and the polar circles (Arctic and Antarctic) are not the borders where 24-hour day or 24-hour night begins. Arctic Day (24-hour day) starts on summer solstice circa 51 arcminutes (latitude) below Arctic Circle, Antarctic Night (24-hour night) starts 51 arcminutes (latitude) below Antarctic Circle. That's because of atmospheric refraction (deviation of light from a straight line) and the size of the Sun. Sunrise occurs when the Sun's upper limb, rather than its center, appears to cross the horizon. Similarly, the sunset occurs when upper limb of the Sun hides over the horizon. These phenomena give us a little longer daytime (7 minutes) on the Equator, bigger Arctic Day Circle and smaller Antarctic Night Circle.See also: Shortest Day of the Year