Conditional Dropdown (formula)

I am trying to get dropdown options to change based on data entered in previously in the row.

I have the formula I need to pull the information in earlier in the row but at the end of the sheet, I have columns with dates and need the option to pull that value over to only the cells that are selected. This seemed like a drop down would be best, but I am open to anything.

Example: Column 3 has the option of being A-G and pulls in the correct letter with a formula based on another sheet.

Columns 4-28 are all dated where I need the letter (A-G) that was populate via formula in column 3 to be put into only the ones I select. therefore column 5,7,8, and 10 need to have "B" in them while the rest of the columns in this row remain blank/unselected

Name     Category      Letter       Date 1     Date 2     Date 3     Date 4      Date 5      Date 6     Date7 -->

test         Project         B                                B                              B              B                               B

test2      Project           G                G                                                                              G

Is this possible?

Thanks!