This study is a 2-cohort, open-label, multicenter, phase 2 study of a short course of immunotherapy consisting of sequential decitabine followed by pembrolizumab administered prior to a standard neoadjuvant chemotherapy regimen for patients with locally advanced HER2-negative breast cancer. The primary efficacy objective is to determine if the immunotherapy increases the presence and percentage of tumor and/or stromal area of infiltrating lymphocytes prior to initiation of standard neoadjuvant chemotherapy. Efficacy will be evaluated in 2 cohorts based on hormone receptor status.