We present a combinatorial and computational commutative algebra methodology for
studying singularities of Schubert varieties of flag manifolds. We define the combinatorial
notion of *interval pattern avoidance*. For "reasonable" invariants P of singularities, we
geometrically prove that this governs (1) the P-locus of a Schubert variety, and (2) which
Schubert varieties are globally not P. The prototypical case is P="singular"; classical
pattern avoidance applies admirably for this choice [Lakshmibai-Sandhya'90], but is
insufficient in general. Our approach is analyzed for some common invariants, including
Kazhdan-Lusztig polynomials, multiplicity, factoriality, and Gorensteinness, extending
[Woo-Yong'05]; the description of the singular locus (which was independently proved by
[Billey-Warrington '03], [Cortez '03], [Kassel-Lascoux-Reutenauer'03], [Manivel'01]) is
also thus reinterpreted. Our methods are amenable to computer experimentation, based on
computing with *Kazhdan-Lusztig ideals* (a class of generalized determinantal ideals) using
Macaulay 2. This feature is supplemented by a collection of open problems and conjectures.