However, a mistranslation that occurred in the first volume of the 1953 New World Translation of the Hebrew Scriptures helps to demonstrate that the translators did indeed know Hebrew.
The NWT is the only Bible translation to have "Jehovah" in Lev 23:21.
In 1953, The first volume of the NWT of the Hebrew Scriptures was published, which had Leviticus 23:21 read:
"And you must proclaim on this very day Jehovah's holy convention for yourselves."
This stood for 26 years, when it was changed to:
"And you must make a proclamation on this very day; there will be a holy convention for yourselves."
Lev. 23:21 has יהיה ("there will be") which is remarkably similar to יהוה ("Jehovah"). The translators misread יהיה as יהוה and later corrected this as noted in the 8/15/79 Watchtower's notice to it's readers on page 31:
In making the New World Translation of Leviticus 23:21, the Hebrew יהיה (“it will be”) was misread as יהוה (“Jehovah”). Hence, the first sentence of this verse should read: “And you must make a proclamation on this very day; there will be a holy convention for yourselves.”'
The original mistake definitely shows that the translators were either working from or comparing a Hebrew text and not simply “revising” English bibles, and the revision shows that the later revisers were also checking the Hebrew.