Which are the specifics of a memory that are lost first?
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.