Additional Study Information: The goal of this study is to determine if ribociclib in combination with letrozole as neoadjuvant endocrine therapy increases the proportion of women with PEPI score of 0 at surgery, compared to patients treated with letrozole alone therefore allowing more patients excellent outcomes without chemotherapy. The PEPI score measures the risk of breast cancer returning in women who received endocrine (hormone) therapy before for surgery. This technique uses tumor size, cancers proximity to lymph nodes, hormone receptor status of the cancer, and Ki67 (a cellular protein) blood levels to calculate this risk.