Hello
You can use a periodic lot sizing procedure (PK, for example) and define a planning calendar on transaction MD25 with the desired dates. MRP will only create purchase requisitions on the periods defined on the planning calendar.
Take a look on the following links for more details:
Period Lot-Sizing Procedures - Material Requirements Planning (PP-MRP) - SAP Library
Planning Calendar - Material Requirements Planning (PP-MRP) - SAP Library
BR
Caetano