The steps to accomplish so are as follows:
Select all of the cells that have dates for which the month name should be displayed.
Go to the Home tab.
Click the dialogue box launcher icon in the Number group (or use the keyboard shortcut Control +1). The Format Cells dialogue box will appear.
Select Custom from the Category drop-down menu.
Enter'mmmm' in the type field. In the Sample preview, you should see one of the month names.
All of the dates would be converted to their complete month names using the processes above (as shown below).