Objectively, Derek knew that days of the week were just days of the week. They could be neither good, nor evil. Morality did not apply to them.
They just were.
But even with all of that, he had to admit one thing to himself. For him, Thursdays were good days.
At first his love for Thursdays, and him labelling them a good days had come from the fact that on Thursdays, he and Emily slept together.
Thursdays meant that after hours upon hours of being awake. Derek got to lay down with Emily by his side and go to sleep. Dreaming like all other people did, and then waking up in the morning ready to start the day.
But now the sleep bit was being moved backwards a bit, into the second position. A very distant second for that matter. There was now another reason for Derek to love Thursdays even more. And that reason was that Emily had completed the majority of the puzzle. Now all that Derek had to do was slot in, the final peace that he held.