Outlook 2010 has a feature which when enabled would automatically capitalize first letter in names of days i.e if you enter “monday” it will be changed to “Monday”. This is a quite useful feature but if you do want to capitalize names of days in Outlook then you can do the following.
Click the File menu then the Options link. In the Outlook Options window, navigate to Mail Option and click Spelling and Autocorrect button.
Click the AutoCorrect Options button available under the Proofing section..
Un tick the check box with label as Capitalize names of days available under the AutoCorrect tab.
Click the OK button to confirm and save the changes.