Longest Day of The Year

June 20th, 2016 - Summer Solstice. First day of summer in northern hemisphere.
Place mouse cursor over the image to see latitude and length of daytime in any location of the World during summer solstice - June 20th, 2016
Figure 1 (left) - Shadowed World map in Mercator projection. The darker the area the shorter the day. Figure 2 (right) - latitude vs. length of daytime relation plot.
Sources: Mercator map - wikipedia, author: Strebe; shadow overlay, latitude-daytime script and figure 2 - time.unitarium.com
Table 1 Exact dates and times (UTC) of equinoxes and solstices.

Circle of Latitude Latitude Daytime
Arctic Circle 66° 33′ 44″ N (66.5622°) 24:00
Arctic Day > 65° 43' 04″ N (65.7173°) 24:00
Tropic of Cancer 23° 26′ 16″ N (23.437778°) 13:35
Equator 12:07
Tropic of Capricorn 23° 26′ 16″ S (-23.437778°) 10:42
Antarctic Circle 66° 33′ 44″ S (-66.5622°) 02:10
Antarctic Night >67° 24′ 51″ S (-67.4086°) 00:00
Table 2 - Length of daytime on the Circles of Latitude, Arctic Night and Antarctic Day circles

When is the longest day of the year?

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 2016 summer solstice is observed on june 20th, exactly at 22:34 UTC. This is also the day of the shortest day and the longest night in the Southern Hemisphere. 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.

How long is the longest day of the year?

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 'how long is the longest day in the year', the longest day on the Earth lasts 24 hours, but more accurately, you could say - 'it depends on latitude'.

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 Longest Day of the Year in New York City, US

Some interesting facts (e.g. Why the day on the Equator is longer than 12 hours?)

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, Earth Illumination Now,
Rio 2016 Schedules in Your Local Time, Time difference between You and Rio de Janeiro

Table 3 - Cities around the World, sunrise and sunset times, day length, times of civil, nautical, astronomical twilight during Summer Solstice, June 20th, 2016. Sorted by latitude. All times are local.
Location Day Civil Twilight Nautical Twilight Astronomical Twilight
City Country Latitude Longitude Sunrise Sunset Day Length Begin End Length Begin End Length Begin End Length
ThuleGreenland77° 29' 21''-69° 19' 55''NoneNone24:00NoneNone24:00NoneNone24:00NoneNone24:00
TromsoNorway69° 40' 00''18° 58' 00''NoneNone24:00NoneNone24:00NoneNone24:00NoneNone24:00
FairbanksUS, AK64° 54' 02''-148° 09' 39''02:5600:5221:56NoneNone24:00NoneNone24:00NoneNone24:00
AnchorageUS, AK61° 09' 49''-149° 59' 31''04:2123:4219:21NoneNone24:00NoneNone24:00NoneNone24:00
St.PetersburgRussia59° 53' 39''30° 15' 51''04:3523:2518:5002:5201:0922:17NoneNone24:00NoneNone24:00
MoscowRussia55° 45' 07''37° 36' 56''04:4422:1817:3403:4323:1919:36NoneNone24:00NoneNone24:00
LondonUK51° 30' 00''0° 07' 34''04:4321:2116:3803:5522:0918:1402:4023:2320:43NoneNone24:00
New York CityUS, NY40° 45' 10''-73° 58' 21''05:2420:3015:0604:5121:0316:1304:0821:4617:3803:1722:3719:19
BeijingChina39° 55' 44''116° 23' 16''04:4519:4615:0104:1320:1916:0603:3121:0017:2902:4221:4919:07
TokyoJapan35° 41' 06''139° 45' 03''04:2519:0014:3503:5519:3015:3503:1720:0716:4902:3620:4818:12
Los AngelesUS, CA34° 03' 10''-118° 15' 50''05:4120:0714:2605:1220:3615:2404:3621:1216:3603:5721:5217:55
MiamiUS, FL25° 48' 44''-80° 14' 24''06:3020:1513:4506:0420:4114:3705:3221:1215:4004:5921:4516:46
HonoluluUS, HI21° 19' 15''-157° 50' 20''05:5019:1613:2605:2519:4014:1604:5520:1015:1504:2420:4116:16
MumbaiIndia18° 58' 30''72° 49' 32''06:0219:1813:1605:3719:4214:0505:0820:1115:0304:3920:4116:03
SingaporeSingapore1° 17' 35''103° 51' 21''07:0019:1212:1206:3719:3412:5706:1120:0013:4905:4520:2714:42
La ConcordiaEcuador0° 00' 00''-79° 22' 59''06:1518:2212:0705:5318:4512:5205:2619:1113:4505:0019:3714:37
Rio de JaneiroBrazil-22° 54' 00''-43° 13' 59''06:3217:1610:4406:0817:4011:3305:4018:0912:2905:1218:3613:24
Cape TownSouth Africa-33° 55' 00''18° 25' 00''07:5117:4409:5407:2318:1210:4906:5218:4311:5206:2119:1412:53
MelbourneAustralia-37° 49' 00''144° 58' 01''07:3517:0809:3307:0617:3710:3106:3318:1011:3706:0118:4212:41
Puerto WilliamsChile-54° 55' 59''-67° 37' 00''08:5616:0707:1108:1116:5308:4207:2317:4010:1706:3918:2411:46