The 1st transatlantic radio signal was received on December 12, 1901.

On 12 December 1901, Guglielmo Marconi and his assistant, George Kemp, heard the faint clicks of Morse code for the letter “s” transmitted without wires across the Atlantic Ocean. 
Milestones:Reception of Transatlantic Radio Signals, 1901 – Engineering and Technology History Wiki

Canada has issued two stamps celebrating Marconi’s association with the country. The first was released on the anniversary of his birth and the second was part of a Wireless Telegraphy set. Both stamps used the same Marconi image. 

Birth Centenary of Guglielmo Marconi (1874-1937)
Released by Canada post in 1974
Designer: John B. Boyle

This is the graphic proof for the stamp, prior to adding the lettering.

The signal was sent from Poldhu, Cornwall to Marconi’s antennae set up at Signal Hill in St John’s, Newfoundland. The signal was sent by Marconi’s team in Poldhu, who repeatedly sent the “s”  between 3 to 7 pm (GMT).

Wireless Telegraphy, Marconi
Part of theCommunications Technology 
Released in 2002 by Canada Post
Designer: Bonnie Ross and Susan Warr

At the prearranged time on the 11th, Marconi and his assistants sent up a balloon, but heard nothing from their receiver. They next dispensed with the tuned receiver and tried a more sensitive detector, but the balloon broke loose. On the 12th, a strong gale still blew and carried away the first kite they sent up. The second kite, which trailed 500 feet of antenna wire, stayed up long enough for Marconi and Kemp to hear the transatlantic signals through a telephone earpiece connected to the receiver. Marconi’s diary for that date has the simple entry, “Sigs. at 12:30, 1:10 and 2:20. 11 more signals were confirmed on the next day, Friday the 13th, but none on Saturday. On Monday the 16th, Marconi released the news to the press and then began packing for a new location because the Anglo-American Telegraph Company threatened legal action for violating its communication monopoly in Newfoundland.
Milestones:Reception of Transatlantic Radio Signals, 1901 – Engineering and Technology History Wiki