The world’s first adhesive postage stamp, the Penny Black, went on sale May 1, 1840. 
Although some Penny Blacks were sold on May 1, it wasn’t officially released until May 6, 1840. 
1 cover is known to exist stamps May 1, 1840.

Penny Black
Issued by Royal Mail May 1, 1840
Designers/Engravers: Charles and Frederick Heath

When the UK post office was being reformed in 1838, one of the new requirements was the creation of an inexpensive adhesive postage stamp to replace the inefficient tariff system. The Treasury department initiated a design competition for a penny post stamp (and cover).  Over 2,600 entries were submitted, including over 100 by designers James Chalmers and Charles Whitting alone. Their designs are among the few that have survived.

None of the 2,600 designs were approved by the men in charge of the new postal system Sir Rowland Hill and his assistant Sir Henry Cole. Hill was a vocal proponent of having the Queen appear on the first stamp. So after rejecting the thousands of entries, Hill and Cole decided artist Henry Corbould’s sketch of young Queen Victoria would be used.

there is nothing in which minute differences of execution are so readily detected as in a representation of the human face … I would therefore advise that .. a head of the Queen by one of our first artists should be introduced.
Printing British Postage Stamps, Douglas N. Muir, British Postal Museum archives

Corbould based his work on William Wyon’s 1837 medallion, Silver Wyon Medal or City Medal.   

The medal’s image was originally sculpted by Wyon in 1834 and it portrays a 15-year-old Princess Victoria. By 1838, Elizabeth was 20 years old, but the Wayon image was the one the public would soon be most familiar with.  

Interestingly, the image of Queen Victoria which was used on Penny Black was based on a sketch of her aged 15. However, it was to remain on stamps for the entirety of her reign – which lasted almost 64 years until she died at Osborne House on the Isle of Wight, on 22 January 1901. 
The ULTIMATE Penny Black stamp guide (value, history, rarity & legacy) (warwickandwarwick.com)

The design was signed off by Hill in late 1839 and sent off to father and son engravers Charles and Frederick Heath. The Heaths prepared a number of essays and plates that dealt with the final image, background, postage amount and outlines before the now familiar Penny Black was born.  

The Penny Black was in print for less than a year before it was replaced in 1841 with the Penny Red (which used the Black plates). 68,808,000 Penny Blacks were printed, using 11 plates. The plate, ink and cancel variations alone have created an entire industry devoted to collecting the Penny Black. Some of the variations were the result of the plates being worn down from repeated use and various repairs that created small differences. 

To further complicate collecting, each sheet of stamps included numbers and letters: “Each sheet contained 240 letter combinations, starting with AA at the top left and going down to TL at the bottom right.”(The ULTIMATE Penny Black stamp guide (value, history, rarity & legacy) (warwickandwarwick.com).  Within each sheet, more variations can be found, even when the same plate was used.  

In fact, a total of 11 different plates were produced and the first one had to be so extensively renovated that specialists recognise two versions of it, plate 1a and plate 1b, the other ten plates, plates 2 to 11 make a total of 12, so an early target might be to collect one stamp from each, with number 11 likely to be the most expensive – unless you are very lucky!

The plates can be identified because the corner letters were individually ‘punched’ into them and no two pairs of letters are in exactly the same positions in their corner squares. That, plus the possibility of identifying other marks on the stamp, allows more experienced collectors to ‘plate’ a Penny Black, possibly finding a scarce plate 11 which someone has identified as one of the more common ones. How to collect Penny Blacks | Stanley Gibbons

The first press run of Penny Blacks happened April 15, 1840, using plates 1a and 1b. These two plates printed the greatest number of stamps, 10,052,400 (warwickandwarwick.com).

If you’d like to read more about the plates and numbering, Warwick and Warwick has a great run down on the combinations. If you are ready for a slightly more in depth look at “plating”, head over to British Empire Stamps: British Stamp Collections, Hampshire, South, UK: John Lamonby. 

Oh, and Queen Victoria was reported to have expressed “high appreciation” of the new stamp.