You plan an activity that starts on February 1 and finishes on February 28.

To define a milestone for February 10 that should not be changed through scheduling, you should enter February 10 as fixed date for the milestone. A fixed date is a date that remains unchanged during scheduling, regardless of any changes in the dates of preceding or succeeding activities or milestones. A fixed date can be entered manually or automatically derived from a constraint. Reference:https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html