Which are the specifics of a memory that are lost first? Background:
In the last few days, I came across an experience that made me wonder how our brain manages "memory resources". 

As time progresses further from an experience I've had in my life, I realize that I have begun to forgot details that are important and supposed to be "kept", not lost since I was able to recall them in the first days following the event.

I remember what happened, who was present, and why it happened, but I'm beginning to lose track of "when" the event happened.

This may be a filtering process or a defense mechanism, but in any case, I'm not focusing on the reason behind that, rather I'm interested in avoiding such "holes" in memory in the future.

In a generalized way, given the facts that we can obtain by answering the questions "what", "why", "who" and "when", which of these are the first facts to be forgotten?
I was taught that recall and forgetting are somewhat worse for episodic memories as compared to semantic memories. I'm having trouble finding a supportive reference now, but that may be due to how basic the concept is (or it may just be wrong). The example I recall was the difference between remembering how to ride a bike vs. remembering the experience of learning to ride a bike – one expects more people to remember how to ride a bike than to remember learning how. This isn't the cleanest example of two different kinds of memories, because knowing how to ride a bike is also more likely an implicit, procedural memory.

In any case, forgetting "when" seems likely if this entails the circumstantial aspects of the event. If it's not especially important that the event in memory happened at a particular time or in a particular situation, those elements of the episodic memory could be forgotten quite readily. I don't think "when" information is categorically forgettable though, as I can imagine some memories being tied very closely to that info – e.g., memories about parties on holidays or birthdays.