Add vietnamese holidays

src/Calendars/ChineseCalendar.php

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+<?php
+
+namespace Spatie\Holidays\Calendars;
+
+use Carbon\CarbonImmutable;
+use DateTimeZone;
+use IntlDateFormatter;
+
+trait ChineseCalendar
+{
+    protected string $timezone = 'Asia/Shanghai';
+
+    public function setTimezone(string $timezone): static
+    {
+        $this->timezone = $timezone;
+
+        return $this;
+    }
+
+    protected function chineseToGregorianDate(string $input, int $year): CarbonImmutable
+    {
+        $timestamp = (int) $this->getFormatter()->parse($year . '/' . $input);
+
+        return (new CarbonImmutable())
+            ->setTimeStamp($timestamp)
+            ->setTimezone(new DateTimeZone($this->timezone));
+    }
+
+    protected function getFormatter(): IntlDateFormatter
+    {
+        return new IntlDateFormatter(
+            locale: 'zh-CN@calendar=chinese',
+            dateType: IntlDateFormatter::SHORT,
+            timeType: IntlDateFormatter::NONE,
+            timezone: $this->timezone,
+            calendar: IntlDateFormatter::TRADITIONAL
+        );
+    }
+}

src/Countries/Vietnam.php

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+<?php
+
+namespace Spatie\Holidays\Countries;
+
+use Carbon\CarbonImmutable;
+use DateTimeZone;
+use IntlDateFormatter;
+use Spatie\Holidays\Calendars\ChineseCalendar;
+
+class Vietnam extends Country
+{
+    use ChineseCalendar;
+
+    public function countryCode(): string
+    {
+        return 'vn';
+    }
+
+    protected function allHolidays(int $year): array
+    {
+        return array_merge([
+            // New Year's Day
+            'Tết Dương Lịch' => '01-01',
+            // Reunification Day
+            'Ngày Giải Phóng Miền Nam, Thống Nhất Đất Nước' => '04-30',
+            // Labour Day
+            'Ngày Quốc Tế Lao Động' => '05-01',
+            //A day before the Independence Day
+            'Ngày Trước Quốc Khánh' => '09-01',
+            // Independence Day
+            'Ngày Quốc Khánh' => '09-02',
+            //A day after the Independence Day
+            'Ngày Sau Quốc Khánh' => '09-03',
+        ], $this->variableHolidays($year));
+    }
+
+    /** @return array<string, CarbonImmutable> */
+    protected function variableHolidays(int $year): array
+    {
+        $this->setTimezone('Asia/Ho_Chi_Minh');
+
+        return [
+            // Lunar New Year's Eve
+            'Ngày Ba Mươi Tết' => $this->chineseToGregorianDate('12/30', $year - 1),
+            // Lunar New Year Day 1
+            'Mùng Một Tết Âm Lịch' => $this->chineseToGregorianDate('01/01', $year),
+            // Lunar New Year Day 2
+            'Mùng Hai Tết Âm Lịch' => $this->chineseToGregorianDate('01/02', $year),
+            // Lunar New Year Day 3
+            'Mùng Ba Tết Âm Lịch' => $this->chineseToGregorianDate('01/03', $year),
+            // Lunar New Year Day 4
+            'Mùng Bốn Tết Âm Lịch' => $this->chineseToGregorianDate('01/04', $year),
+            // Hung Kings' Festival
+            'Ngày Giỗ Tổ Hùng Vương' => $this->chineseToGregorianDate('03/10', $year),
+        ];
+    }
+}

tests/.pest/snapshots/Countries/VietnamTest/it_can_calculate_vietnamese_holidays.snap

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+[
+    {
+        "name": "T\u1ebft D\u01b0\u01a1ng L\u1ecbch",
+        "date": "2024-01-01"
+    },
+    {
+        "name": "Ng\u00e0y Ba M\u01b0\u01a1i T\u1ebft",
+        "date": "2024-02-09"
+    },
+    {
+        "name": "M\u00f9ng M\u1ed9t T\u1ebft \u00c2m L\u1ecbch",
+        "date": "2024-02-10"
+    },
+    {
+        "name": "M\u00f9ng Hai T\u1ebft \u00c2m L\u1ecbch",
+        "date": "2024-02-11"
+    },
+    {
+        "name": "M\u00f9ng Ba T\u1ebft \u00c2m L\u1ecbch",
+        "date": "2024-02-12"
+    },
+    {
+        "name": "M\u00f9ng B\u1ed1n T\u1ebft \u00c2m L\u1ecbch",
+        "date": "2024-02-13"
+    },
+    {
+        "name": "Ng\u00e0y Gi\u1ed7 T\u1ed5 H\u00f9ng V\u01b0\u01a1ng",
+        "date": "2024-04-18"
+    },
+    {
+        "name": "Ng\u00e0y Gi\u1ea3i Ph\u00f3ng Mi\u1ec1n Nam, Th\u1ed1ng Nh\u1ea5t \u0110\u1ea5t N\u01b0\u1edbc",
+        "date": "2024-04-30"
+    },
+    {
+        "name": "Ng\u00e0y Qu\u1ed1c T\u1ebf Lao \u0110\u1ed9ng",
+        "date": "2024-05-01"
+    },
+    {
+        "name": "Ng\u00e0y Tr\u01b0\u1edbc Qu\u1ed1c Kh\u00e1nh",
+        "date": "2024-09-01"
+    },
+    {
+        "name": "Ng\u00e0y Qu\u1ed1c Kh\u00e1nh",
+        "date": "2024-09-02"
+    },
+    {
+        "name": "Ng\u00e0y Sau Qu\u1ed1c Kh\u00e1nh",
+        "date": "2024-09-03"
+    }
+]

tests/Countries/VietnamTest.php

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+<?php
+
+namespace Spatie\Holidays\Tests\Countries;
+
+use Carbon\CarbonImmutable;
+use Spatie\Holidays\Countries\Vietnam;
+use Spatie\Holidays\Holidays;
+
+it('can calculate vietnamese holidays', function () {
+    CarbonImmutable::setTestNowAndTimezone('2024-01-01', 'Asia/Ho_Chi_Minh');
+
+    $holidays = Holidays::for(country: 'vn')->get();
+
+    expect($holidays)
+        ->toBeArray()
+        ->not()->toBeEmpty();
+
+    expect(formatDates($holidays))->toMatchSnapshot();
+});