You want to make sure that certain tasks must start on a given date.

To maintain constraints for start dates, you can use activities. Activities are structural elements of a network that represent tasks or partial tasks within a project. Constraints are parameters that define fixed dates or time intervals for activities or networks. Constraints can be used to influence the scheduling of project objects and to ensure that certain deadlines are met. You can maintain constraints for start dates or finish dates of activities or networks in the project builder or the project planning board. Reference:https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html