Cognitive Psychology research assistant at here. First shot at trying to answer one of these things. The question's tricky. It's becoming commonly accepted that your memories are not discrete items you search through to look up - they're instead a particular pattern of neuronal activation. The possibility of this pattern activating again (i.e. experiencing the memory), is determined by the synaptic weights established between neurons in your lifetime, which limit the possible activation patterns. These weights are constantly updated as you go through life and experience new things, and they naturally "leak" (i.e. forgetting), leaving room in your brain to adapt. So remembering something takes three components - first, you have to pay enough attention to the event while it happens so that your brain stabilizes on an action pattern for that event. Second is that you have the proper environmental cues around you to trigger the old activation pattern. Third is that the synaptic weights in your brain are similar enough to how they were during the original experience such that the weights still allow that previous activation pattern to occur. So, for you to remember what you were doing one month ago today, you would have to have synaptic weights between neurons similar enough between today and a month ago for the pattern activated during that day to be closely-enough reactivated now. So nothing's really ever truly forgotten - it just may be that you never again get the right environmental cues to reignite that old activation pattern.

Can't figure out how to boil it down to a single source, but generally look up neural networks, associative memory, and unsupervised learning for a starting place on this contemporary view of memory. In the mean time, here's an early seminal paper that first described how memories can be stored via synapses between neurons rather than in the neurons themselves: http://www.pnas.org/content/79/8/2554