one of these days

but never 

two of these days

or

three of these days

or

four of these days

Fermat's Last Theorem states that are no three positive integers x, y, and z that satisfy the equation x^n + y^n = z^n for any integer value of n greater than 2.

In other words, the Pythagorean Theorem, a^2 + b^2 = c^2 doesn't apply to higher exponent numbers. There is no way to make a^3 + b^3 = c^3.

In Goodstein's Only Theorem, there is no number n (greater than 1) that makes the phrase n of these days make sense.