I am currently working with a pivot table someone else created. The data set has dates only, in the correct format. The slicer has the dates categorized by month and excludes the dates in which there was no data.

Here is my problem: I added a few new months of data. Everything is calculating just fine in the Pivot table. However, my slicer where I should be able to pick the month now has these months listed as all their individual dates. I want to put the December dates under "December" and so on. The data that was present before I loaded anything additional still has this feature.

How do I group these dates, to be able to them as months? By the way, the grouping function is grayed out.