A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
If your months are separate sheets, I would group them and type the entry once.
Hold Ctrl and click each month sheet tab, or click the first month tab, hold Shift and click the last month tab. Click the cell for that date and type the entry. Excel puts the same entry in the same cell on every grouped sheet.
Right-click a sheet tab and choose Ungroup Sheets right after that, before you edit anything else. Any change you make while the sheets are grouped applies to all of them.
This only works if the same date is in the same cell on each month sheet. If all 12 months are on one sheet, copy the entry and paste it into the matching date cell for each month instead.