Partition a table at creation
This is obviously opinion based, but I'd go ahead and partition the table at creation time.  I'd recommend RANGE RIGHT, as you apparently want to partition on a DATE and make sure you pre-define enough partitions before the minimum date you currently have and after the maximum date you currently have.  This will mitigate massive data movement when adding additional partitions in the future.

Check out Dan Guzman's Table Partitioning Best Practices