Getting days count from start day and number of day for a group of days in Java 8
You came far to a good solution.

public void getDayCount(List&lt;DaySpan&gt; daySpans){
    Map&lt;LocalDate, Long&gt; freqs = daySpans.stream()
            .flatMap(daySpan -&gt;
                IntStream.range(0, daySpan.numberOfDays)
                    .mapToObj(i -&gt; daySpan.date.plusDays(i)))
            .collect(
                Collectors.groupingBy(Function.identity(), Collectors.counting()));

    freqs.entrySet().stream()
            .sorted(Map.Entry::getKey)
            .forEach(System.out::println);
}



Creating a Stream&lt;LocalDate&gt; for some DaySpan is simply realized by an IntStream.range.
A flatMap makes a larger Stream of LocalDates.
Turning them into a counting map you already did.
Sorting them on the date remains.
You printed the entries as Entry, so I kept it that way.