Which season?

Sherlock: But the evidence is overwhelming my dear Watson!
Dr Watson: In what way?
Sherlock: Surely you can see the robin?
Dr Watson: Yes
Sherlock: And what is it sitting on?
Dr Watson: I think it’s holly
Sherlock: And when do you get robins and holly Watson?
Dr Watson: Winter I think
Sherlock: Elementary my dear Watson, elementary!