I thought it was in 1914. Silly me!
w78 3/1 p. 18 par. 15 - When was God’s Son made a King-Priest like Melchizedek? This was 40 days after his resurrection from the dead, when he ascended to heaven.
it-2
p. 711 - Upon his ascension to heaven, he was seated at God’s
right hand, (as king???) waiting until his enemies would be placed as
a stool for his feet. He also became a priest according to the manner
of Melchizedek. (wasn't he a king–priest???)