I would say it depends on the number of dates and events, as well as which one will act as the cue to retrieve the other. If you have a lot of different events to memorize that fall under the years 1939 and 1940, and need to discern between these dates when cued with the event, then it’s probably best to create two discrete architectural units and then populate them with the events. So, for instance, the first floor of a house would be 1939, and you populate it accordingly, and the second floor would be 1940 and populated accordingly. In this case, you can imagine Hitler at your front door doing something silly. When you hear “Hitler”, you recall this imagine of him at the front door, and know it’s 1939.
It would be more difficult if you have dozens or hundreds of different events and dates. In this case, I’d organize the palace into units by century (19th, 20th, etc), perhaps one room or one group of rooms by century. This gives you the benefit of only needing to encode two numbers, so for 1939 you only have to encode “39”. The benefit here is that it gives you wonderful creative ability in encoding dates and events. In the 20th century, you can see Hitler holding an object and doing an action that encodes 39, and then you know Hitler -> 1939. Or, you can see a Person with an Object doing a Hitler salute, or something else. You can be even more precise if you wish, and break the room(s) into part by decade, so you only have to encode the “9” in of 39, and you know it belongs to the third decade because it’s in the 3rd quadrant of the room.
By far the best course of action would be to organize the walk chronologically, so you can walk through the years and events. This is something that the author Mark Twain actually did, with stakes on his large property. He had a walking path and place a stake down for each English monarch, the distance of the stake being the number of years in feet, and then would imagine the monarchs on those stakes. He liked this so much that he spent a year trying to sell this idea as a game for children, which of course failed, because children don’t want to memorize every English monarch.