I answered this question on Quora some time ago: "Why do I feel that there are some days that pass so fast and other days too slowly?"
I'll repost my reply here:
Human beings reduce familiar actions and things to abstract models. We don't fully notice an individual chair once we mentally assign it to the class of "chairs". They almost "disappear" into their categories.
As we grow older and experience more and more, fewer and fewer things surprise us. Nearly everything is categorized, and thus falls beneath our conscious awareness. Eventually, we're no longer living in the world; we're living in "Worldworld", a set of assumptions and summations and mental shortcuts that keep us from experiencing the here-and-now in a fresh and vital way.
This is adaptive; we need to be on guard for the new and the mysterious (i.e. stuff that might harm us), not pouring our attention into every single daisy the way young children do. But this takes us out of our senses and into our heads (our abstraction-oriented cortex), and time passes much more quickly when attention is focused there, on symbols, shortcuts, and taxonomies, rather than on an infinitely rich and nuanced array of ever-shifting perceptions. That's why time appears to go faster as we age. Less surprises us, and more is reduced to mental abstraction. The world disappears as the model fills out.
Days which pass slowly are ones where you, for whatever reason, "come back to your senses" and experience the raw immediacy of your surroundings. Because such experience is richer/denser, the pace of time appears to slow. This happens when we're thrust into a new or surprising environment, or when some internal impulse spurs you to temporarily discard the mental modeling and immerse in the immediate.
Immersing in the immediate sounds lovely, and it can be. But it can also result in a cognitive stall - aka boredom. Everything seems immediate - but oppressively so! That's a bad sort of slow-passing day. So...experience of the immediate is not always lovely. And, by the same token, to focus attention on abstract mental modeling can be profound - especially if you're a mathematician! Neither perspective is fundamentally better or worse. But if time seems to uncomfortably slip away, you need to shift from abstraction to perception. And if time seems to uncomfortably drag, you need to shift from tedious over-awareness of your prosaic surroundings and find the infinite space and freedom in your creative imagination.
My other Quora replies.