Harry spent most of the Christmas holidays with Cho.
During that time, apart from briefly supporting Cho on Christmas Day when she was a little weak from using too much magic, Harry had not even touched her hand. Even so, he felt thoroughly invigorated.
He simply felt especially happy every day.
The clearest sign was that his Patronus had become considerably brighter. Given Harry's current reserves of magic, there was no way they could have increased rapidly in just over a week.
Such a marked improvement in the spell's effect could only mean that his mental state, emotions, and thoughts had become more attuned to it. After all, both white magic and dark magic were linked to emotions.
With Harry brimming with positive energy lately, it was perfectly reasonable for his Patronus Charm to grow stronger.
His time with Cho was so enjoyable that Harry completely forgot he had another reason for staying at Hogwarts-to study a more advanced form of the Patronus Charm, the Corporeal Patronus, with Professor Lupin.
It had slipped his mind entirely. Even practicing the Patronus Charm with Cho almost every day had failed to remind him.
Not until the evening of the ninth day of the holidays did he receive a note from Professor Lupin.
"Dear Potter, I hope this note finds you well. I have prepared a Lethifold for your Patronus practice, but I wonder whether, busy as you are studying and researching magic with a classmate, you are still interested in further study of the Corporeal Patronus? If you need me, I have been spending my time in my office lately.-Remus Lupin"
At that moment, Harry's face turned even redder than it had while practicing flying and magic with Cho during the day on Christmas Eve.
It was not only because studying magic