Routine makes many days blend together, while novelty gives memory more distinct details to hold on to.
Many ordinary days are difficult to distinguish from one another after they have passed. The routine is useful while we are living it, but it leaves few markers in memory. A new place, an unfamiliar task, or an unexpected change can make a day easier to recall later.
That may be why some stretches seem full of memories while others feel blank, even if both contained plenty of activity. Novelty gives the mind something different to organise around. I do not think every day needs to be unusual; routine has value too. But it helps explain why the days we remember are not always the busiest ones.