Why do some days feel fast and others feel slow? As per the title, why is it some days it just feels like a day can go so fast, yet other days can feel slow? Every day has the same amount of hours, minutes and seconds after all. What is the actual science behind our state of mind when we feel like this?
Several factors are proven to affect our experience of time. Those that differ from one day to the next include:


Biochemistry affects our perception of time.


  Stimulants produce overestimates of time duration, whereas depressants and anesthetics produce underestimates of time duration.
  http://en.wikipedia.org/wiki/Time_perception

Fear affects our perception of time:


  Hammond points to a study in which people with arachnophobia were asked to look at spiders — the very object of their intense fear — for 45 seconds and they overestimated the elapsed time. The same pattern was observed in novice skydivers, who estimated the duration of their peers’ falls as short, whereas their own, from the same altitude, were deemed longer
  
  When a person is scared, a brain area called the amygdala becomes more active, laying down an extra set of memories that go along with those normally taken care of by other parts of the brain.
  
  "In this way, frightening events are associated with richer and denser memories," Eagleman explained. "And the more memory you have of an event, the longer you believe it took." 
  http://www.brainpickings.org/2013/07/15/time-warped-claudia-hammond/
  http://www.livescience.com/2117-time-slow-emergencies.html
  http://en.wikipedia.org/wiki/Time_perception

Our attention affects our perception of time:


  Wearden et al. (2014) administered a questionnaire which asked about passage of time in different drug and emotional states, and generally found that situations associated with “distraction” from time were associated with faster passage of time, in accord with the idea that “attention to time” is an important determinant of time judgements. 
  http://www.mind-and-brain.de/postdoctoral-program/scientific-events/archive/time/

Having fun(or not):


  When this was tested by Kellaris (1992), they found that when listeners enjoyed the music more, time seemed to slow down.
  
  Or—prepare yourself for a 180 degree about-face—it could all be the other way around. Perhaps you’re having fun when time flies. In other words, we assume we’ve been enjoying ourselves when we notice that time has passed quickly.
  
  There’s evidence for this in a recent experiment by Sackett et al. (2010). Participants doing a boring task were tricked into thinking it had lasted half as long as it really had. They thought it was more enjoyable than those who had been doing exactly the same task but who hadn’t been tricked about how much time had passed.

Being tired:


  When we’re tired, though, our perception of time goes awry and we find it more difficult to distinguish between short spaces of time. This fact can be used to measure whether people are too tired to fly a plane, drive a truck or be a doctor. Indeed just such simple hand-held devices that quickly assess your tiredness are already being developed (Eagleman, 2009).
  http://www.spring.org.uk/2011/06/10-ways-our-minds-warp-time.php

Emotional self-regulation:


  The effort of trying to either suppress or enhance our emotional reactions seems to change our perception of time. Psychologists have found that when people are trying to regulate their emotions, time seems to drag on.
  
  Vohs and Schmeichel (2003) had participants watch an 11 minute clip from the film Terms of Endearment. Some participants were asked to remain emotionally neutral while watching the clip and others were told to act naturally. Those who tried to suppress their emotions estimated the clip had lasted longer than it really had.
  http://www.spring.org.uk/2011/06/10-ways-our-minds-warp-time.php

Body temperature:


  Experiments have found that when body temperature is raised our perception of time speeds up (Wearden &amp; Pento-Voak, 1995). Conversely when we are cooled down, our sense of time also slows down.
  http://www.spring.org.uk/2011/06/10-ways-our-minds-warp-time.php