But that's the reason for using Project. If you think you already know all the dates, why not just put everything in a spreadsheet in Excel?
The correct way to use (and get the benefit from) Project is to first enter all the activities (tasks) required to accomplish your end goal (i.e. a product or service). Second, enter the estimated time span for each activity into the Duration field. Note, the duration is NOT the amount of work required to complete the task but the time span to complete the task. Third, connect the tasks in a logical sequence of performance via the Predecessor field. Most links will be finish-to-start but other variations might also be appropriate. (start-to-start, finish-to-finish, lead, lag). Given those entries, Project calculates all task start and finish dates (and times) based on the Project calendar. If the resulting plan doesn't meet the desired end date, then adjust tasks, durations and/or task links. After that is done, assign resources to each of the tasks. Most likely that will change the plan such that further adjustment is needed.