Speaker meaning must differ from word meaning or there is no metaphor.
I don't see what the speaker could have meant by saying
the heat of a summer day other than the meaning of the words
the heat of a summer day, as in
The heat of a summer day can certainly make me feel lazy.
Therefore, it's not a metaphor as it stands, although perhaps some
context could be created around those words which would make them
metaphoric, as in
Lovely Lucy is the heat of a summer day.
CJ