This protocol is a randomized, phase 2 "window of opportunity" trial assessing the biological effects of short term oral vitamin D administration on breast cancer clinical and translational markers in patients awaiting surgery at the Ottawa Hospital. It takes advantage of the current wait times (2-8 weeks) for breast cancer surgery as a "window of opportunity" to rapidly assess biological changes with vitamin D intake.

Inclusion Criteria:

1. Histologically confirmed female primary breast cancer patients whose surgery is planned for the next 2-8 weeks without neoadjuvant therapy as assessed by multidisciplinary team
2. Age ≥18 years
3. Clinically palpable tumour(s) (greater than or equal to 2 cm)
4. Normal serum and urine calcium and serum PTH values at baseline (as defined by Ottawa Hospital)
5. Written informed consent for study

Exclusion Criteria:

1. Patients with recurrent or metastatic breast cancer
2. History of neoadjuvant hormonal therapy, chemotherapy, or radiation therapy in the last 6 months for their breast cancer or any other cancer treatment
3. ECOG performance Status > 2
4. Hypercalciuria on initial baseline urine, defined as Ca/Creatinine Ratio> 1.0
5. Current or previous history of urolithiasis or hyperparathyroidism
6. Abnormal hepatic function according to Ottawa Hospital norms (Total Bilirubin >2x upper limit of normal, ALT/AST >3x upper limit of normal)and/or abnormal renal function (Creatinine > 150 µmol/L)
7. History of granulomatous disease such as tuberculosis or sarcoidosis.
8. Intake of Vitamin D (cholecalciferol) supplement ≥ 2000 IU/day within the last 2 months
9. Inability to comply with a study protocol in the opinion of the investigator (such as abuse of alcohol, drugs or psychotic states).