This article describes how to go from a crosstab with a Date variable...
...to a table with abbreviated month labels:
A crosstab with a Date/Time variable in either the rows or columns which has been aggregated to monthly.
1. Select your table.
2. Go to Properties > RULES on the object inspector.
3. To apply the rule, select the Plus (+) > Browse Online Library > Modify Headers > Abbreviate Month Labels.
4. Set the Months in field to columns or rows (based on where your dates are in the table).
5. Press OK.
Note, if you select Switch rows and columns on your table, the rule will no longer work due to the table dimensions changing. In this case, you will have to right-click your rule in the object inspector > Edit and change the Months in parameter accordingly.