This question is an exact duplicate of:
Rabbi Breitowitz raised this point (listen for approx 2-3 min from here, the point starts at 22:30m) Since the Jewish calendar was not sanctified (with a Rosh Hashana) beyond the year 5999, is it even possible to move into the year 6000 without a duly constituted Sanhedrin declaring appointed times? In other words, if Moschiach doesn't come by the year 6000, are we stuck in the year 5999 because we have no way of moving halachically into the year 6000?
Edit 2: As @DoubleAA states here
That doesn't mean the holidays have to happen. If we don't Mekadesh the Chodesh, maybe the holiday doesn't happen that year (recall it's "mekadesh YISRAEL vehazemannim"). The Ritva (Chullin 101a) actually implies this.
Edit 1: The question above is crisply stated and embedded in this question