Handle multi-day holidays.

Some countries (e.g. Russia, or Vietnam) have holidays which span several
days. Rather than using `holiday.date` from the date-holidays library, we
can check the `.start` (inclusive) and `.end` (exclusive) dates, and use
these to show multiple holiday days on the calendar.

However since these properties return a `Date` representing the start of
the day in that country, rather than an ISO string like `.date` does, we
also need to pass in the current browser `timezone` to the library, so
that we show the correct dates to the user.

src/lib/holidayUtils.ts

@@ -33,14 +33,18 @@ export function getHolidaysForYear(countryCode: string, year: number, stateCode?
     // The date-holidays lib has translations for many holidays, but defaults to using the language of the country.
     // We can pass in the browser's preferred languages (though the lib doesn't fall back, e.g. from `de-AT` to `de`)
     const languages = navigator.languages.map(lang => lang.split('-')[0]);
-    const opts = { languages }
+    // Start/end dates are returned in that country/state's time zone, so we need to provide our time zone to localise
+    const opts = { languages, timezone: Intl.DateTimeFormat().resolvedOptions().timeZone };
     const hd = stateCode ? new Holidays(countryCode, stateCode, opts) : new Holidays(countryCode, opts);
     return hd.getHolidays(year)
         .filter(holiday => holiday.type === 'public')
-        .map(holiday => ({
-            date: new Date(holiday.date),
-            name: holiday.name
-        }));
+        .flatMap(holiday =>
+            // To handle single- and multi-day holidays, we generate a holiday entry for each day in the period
+            Array.from({ length: daysBetween(holiday.start, holiday.end) }, (_, i) => ({
+                date: new Date(holiday.start.getFullYear(), holiday.start.getMonth(), holiday.start.getDate() + i),
+                name: holiday.name,
+            }))
+        );
 }
 
 // Optimize days off to create the longest possible chains