How about "tenor" and "vehicle"?
In my literature class, these are the terms we use when describing the objects in a metaphor, where the "tenor" refers to the object being described and the "vehicle" refers to the image that carries "the weight of comparison," that to which you are comparing the tenor.
In your example, "thee" would be the tenor, and "a summer's day" the vehicle.
Here is an excerpt from Encyclopedia Britannica:
Tenor and vehicle, the components of a metaphor, with the tenor referring to the concept, object, or person meant, and the vehicle being the image that carries the weight of the comparison. The words were first used in this sense by the critic I.A. Richards.