Abemaciclib and Letrozole in Treating Patients With Endometrial Cancer

Inclusion Criteria: * Patients must have a new histologically confirmed diagnosis of endometrioid adenocarcinoma of the endometrium who are candidates for hysterectomy. * Note: Patients with recurrent disease are not eligible. * Patients must be willing to provide archival tumor biopsy slides from the specimen (e.g. endometrial biopsy or dilation and curettage) which diagnosed them with endometrial cancer. * (Please note: Given the amount of tissue obtained from these specimens is often much more than a core needle biopsy, we do not anticipate difficulties with insufficient tissue.) * Patients should be treatment naive. They should not have received any endometrial cancer directed therapy including medroxyprogesterone acetate, aromatase inhibitors, other hormonal treatments or radiation therapy for treatment of endometrial cancer. * Note: Patients can have used oral contraceptives or hormonal replacement therapy provided these were discontinued 28 days prior to trial enrollment. * Eastern Cooperative Oncology Group (ECOG) performance status =\< 2 within 28 days prior to registration for protocol therapy. * Patients must have adequate organ function for all of the following criteria within 28 days of registration. * Absolute neutrophil count (ANC) \>= 1.5 x 10\^9 /L (within 28 days of registration). * Platelets \>= 100 x 10\^9 /L (within 28 days of registration). * Hemoglobin \>= 8 g/dL (within 28 days of registration). * Patients may receive erythrocyte transfusi…