Excel formula for filling another cell

I have four columns in excel. Column A is named "Date" while Column B is named "Day". Column B depends on the value in Column B and I got the Formula for any specific date for column B and that's not my concern anymore.
Column C is designated for formula and is named "Formula".

Column B and C are to be hidden (not a problem).
Column D is Activities.
My concern is I want Column C to not contain any formula but I want it to be filled with values based on another column or cell.
Column B, C, and D are the connected now to each other.
Column C will contain the formula that will fill Column D based on Column B.
Column D are editable cell meaning to say, I will type in some of the cell in Column D while other cell in Column D are automatically filled based on Column B using the formula in Column C.
Here's a scenario.

You will enter formula in Column C.
If B2 is "Sunday" or "Saturday" , then either "Sunday" or "Saturday" will be automatically copied in Column D.
And if Column B are Weekdays, I will type in the Activities I've done in that day.

Is it possible? If so, what formula or Codes should I be using?