I would use Collectors.groupingBy with an adjusted LocalDate on the classifier, so that items with similar dates (according to the compression given by the user) are grouped together.

For this, first create the following Map:

static final Map<String, TemporalAdjuster> ADJUSTERS = new HashMap<>();

ADJUSTERS.put("day", TemporalAdjusters.ofDateAdjuster(d -> d)); // identity
ADJUSTERS.put("week", TemporalAdjusters.previousOrSame(DayOfWeek.of(1)));
ADJUSTERS.put("month", TemporalAdjusters.firstDayOfMonth());
ADJUSTERS.put("year", TemporalAdjusters.firstDayOfYear());

Note: for "day", a TemporalAdjuster that lets the date untouched is being used.

Next, use the compression given by the user to dynamically select how to group your list of items:

Map<LocalDate, List<Item>> result = list.stream()
    .collect(Collectors.groupingBy(item -> item.getCreationDate()
            .with(ADJUSTERS.get(compression))));

The LocalDate is adjusted by means of the LocalDate.with(TemporalAdjuster) method.