To simplify this expression, we can first find a common denominator for all the fractions:

1 - 1/21 = 20/21
1 - 1/28 = 27/28
1 - 1/36 = 35/36
...
1 - 1/1326 = 1325/1326

Now, we can multiply all these fractions together:

(20/21) * (27/28) * (35/36) * ... * (1325/1326)

To simplify this product, we can cancel out common factors in the numerators and denominators:

(20 * 27 * 35 * ... * 1325) / (21 * 28 * 36 * ... * 1326)

The numerator is the product of all the numbers from 20 to 1325, and the denominator is the product of all the numbers from 21 to 1326.

This expression can be simplified further by canceling out common factors, but it would be a very lengthy process to calculate the final value.