Mark all cells on the same rows as another marked cell

Hey, so this is the problem

I'm making research on index values on a market on different weekdays and my problem is this, I have all the values in one column and all the weekdays on another column, I can mark all the weekdays in the column but I want to mark all the column values on the same row to calculate them together!

Example

|Monday||+0,5|
|Tuesday||-0,5|
|Wednesday||+0|
...
...
|Monday||+0,3|

So what I can do now is search and mark all Mondays, but how do I tell excel to mark the corresponding values on the same row, any ideas?