Sorting month by natural sort order
When a field containing month names is added to a PivotTable or to a Slicer the field is sorted in alphabetical order (April, August, etc.) by default. As you may already have discovered this behavior can easily be changed for the PivotTable allowing you to sort the month in calendar order. The workaround for the Slicers is less obvious however. This post describes how you can naturally sort month fields in calendar order.
Sorting month by natural sort order in the Pivot Table
Changing the sort order of the month name when the field is added to either row or column labels of a PivotTable is relatively easy: You can simply leverage Excels auto-sort functionality as shown below, where ascending sort order for the month row label will sort in calendar order.