Whpalmer4 has already given you the answer:

tasks where note contains <txt>

(I can see that you might intuitively think that a <dte> variable was the answer, but what you are looking for is a piece of text within a note. Values supplied for <dte> can only match date fields like those in the due date, start date, date completed columns).

When you are prompted for a value for <txt>, enter your date in the exact format in which it occurs in the text of your notes.