The phrase "open as day" typically denotes something that is very obvious or clear. Some possible synonyms for this phrase might include "clear as crystal," "plain as day," "obvious as the nose on your face," or "blatant." Other options might include "unmistakable," "transparent," "evident," or "undeniable." Still, other possibilities might include "unambiguous," "palpable," "manifest," or "overt." Different synonyms might be more appropriate in different contexts, depending on the tone and style of the writing, as well as the specific connotations that the writer wishes to convey.