The simple term “day” does not contradict the simple term “night”, although they may be conventionally treated as polar opposites. If we agree to treat them that way, then the proposition “It is day” contradicts the proposition “It is night”.
Hegel developed idiosyncratic shorthand ways of talking that may have the misleading appearance of suggesting that he ignored this distinction. In popular references to Hegelian dialectic, it is very common to hear about “contradictions” between so-called opposites. This can lead to massive misunderstanding of what Hegel was really trying to say, especially if one does not realize how concerned he was to deconstruct so-called polarities.
Polarities involve pairs of terms related by classical negation. A is the opposite of B if A = not-B, where “not” satisfies not-not-X = X. In fact, Hegel routinely criticized so-called Understanding for taking such polarities at face value.
In another piece of idiosyncratic shorthand, he talked about a “unity of opposites”. This refers to a sort of conceptual interdependence, not identity in the strict sense.
A single term may be taken as shorthand for many judgments characterizing a thing. Then “contradiction” between two terms actually refers to some contradiction between implications of the associated judgments.
Platonic dialectic in its most canonical form considered in turn the implications of pairs of contradictory propositions, in order to canvas all possibilities. The important part was really the examination of implications. Hegel, too, was far more interested in analyzing extended implications of things than in some dance of polarities.
Hegel’s dialectic — like Aristotle’s — is fundamentally about improving the subtlety of our distinctions, and thus the quality of our reasoning. If we begin with a polar opposition, the intent is to supersede it. As I previously noted, the standard method for superseding a polar opposition for Hegel is to move toward the concrete — i.e., to replace the abstract, “infinite”, classical negation of polar opposition with some suitable finite difference or specific material incompatibility or “determinate negation”, as he liked to call it. (See also Aristotelian and Hegelian Dialectic; Three Logical Moments.)