Apparently Immanuel Velikovsky said on 1953-10-04:

"I expect new evidence from the Minoan Scripts and the so-called
Hittite pictographs. Texts in the Minoan (Linear B) script were found
years ago on Crete and in Mycenae and in several other places on the
Greek mainland. I believe that when the Minoan writings unearthed in
Mycenae are deciphered they will be found to be Greek. I also claim
that these texts are of a later date than generally believed. 'No
"Dark Age" of six centuries' duration intervened in Greece between
the Mycenaean Age and the Ionian Age of the seventh century.'"
http://www.varchive.org/dag/decipher.htm

And he goes on to say:

"When speaking to the Princeton Forum in October 1953 I did not know
that a young English architect was by then on the verge of publishing
the solution to the riddle of the Linear B script. Only six months
passed since my addressing the Graduate Forum, and the April 9, 1954
front page news of The New York Times made known the exciting
performance of decoding Linear B by Michael Ventris."

So... after 1953-10-04 and before 1954-04-09.

The breakthrough was earlier than that. Ventris wrote to Sir John
Myres in mid June 1952 that "the tablets are in GREEK".

Do you have Andrew Robinson's book on Ventris, Marco? He says "the
key period was undoubtedly two or three weeks in late May and early
June 1952."

There is a grid dated 1952-06-18 with 10 consonants and 5 vowels
labelled with phonetic values and more than 40 Linear B signs on it.

The first "publication" appears to be the BBC radio programme
"Deciphering Europe's early scripts" which was broadcast on
1952-07-01.

So what date should be International Linear B Day?
--
Michael Everson * * Everson Typography * * http://www.evertype.com