When is it possible for a constitutional convention of 1789 to occur?

The answer is never. The 1787 Federal Convention was held under the authority of the Articles of Confederation, a completely different form of national law than the Constitution. Under the present Constitution, a convention is limited to "proposing amendments" as "part of" our present Constitution.

As to when such a convention, an Article V Convention authorized by Article V, will occur, the answer is anytime. The public record of applications, which can be read at FOAVC (see related links), shows that 49 states have submitted more 700 applications for a convention. Congress is required to call a convention when two-thirds (34) of the states submit at least one application for a convention call. Therefore, the answer is any time.

Article V

The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.