How to get the first and last day of a month

Short answer

withDayOfMonth(1) gives the first day, TemporalAdjusters.lastDayOfMonth() the last one:

void main() {
    var date = LocalDate.of(2026, 2, 17);
 
    IO.println(date.withDayOfMonth(1));
    IO.println(date.with(TemporalAdjusters.lastDayOfMonth()));
}

Output:

2026-02-01
2026-02-28

With YearMonth

YearMonth describes a whole month and knows its length:

void main() {
    var month = YearMonth.of(2028, 2);
 
    IO.println(month.atDay(1));
    IO.println(month.atEndOfMonth());
    IO.println(month.lengthOfMonth());
}

Output:

2028-02-01
2028-02-29
29

All days of a month

datesUntil() returns a stream of dates:

void main() {
    var month = YearMonth.of(2026, 3);
    long weekends = month.atDay(1).datesUntil(month.atEndOfMonth().plusDays(1))
            .filter(d -> d.getDayOfWeek().getValue() >= 6)
            .count();
 
    IO.println(weekends);
}

Output:

Common mistake

Assuming a month always has 30 or 31 days. Setting the day by hand is risky — February may not have that day:

void main() {
    LocalDate.of(2026, 2, 30);
}

Output:

Exception in thread "main" java.time.DateTimeException: Invalid date 'FEBRUARY 30'

Thanks for reading!

Short answer

With YearMonth

All days of a month

Common mistake

Functions used in this recipe

Thanks for reading!