There are two peshatim in shas on when and how the mashiach comes. The first is that it can only be on leil pesach, that the leil shimurim is the permanent date for all geula. The other opinon is that the geulah can come at any time. A further development * of this explains that the world being rov zakai is cause for immediate arrival of moshiach, but this balance is delicate, one aveira can tip it the other way, and the condition of rov zakai may not be achieved again for perhaps hundreds or thousands of years. Something to think about when a mitzvah presents.
*hagada shehl pesach Bais Haleivi 119