diff --git a/19_counting_sundays/counting_sundays_test.go b/19_counting_sundays/counting_sundays_test.go
new file mode 100644
index 0000000..2de6fbc
--- /dev/null
+++ b/19_counting_sundays/counting_sundays_test.go
@@ -0,0 +1,103 @@
+package main
+
+import (
+	"testing"
+)
+
+func Test_calWeek(t *testing.T) {
+	type args struct {
+		year  int
+		month int
+		day   int
+	}
+	tt := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			"1900-01-01",
+			args{
+				1900,
+				01,
+				01,
+			},
+			1,
+		},
+		{
+			"2020-10-19",
+			args{
+				2020,
+				10,
+				19,
+			},
+			1,
+		},
+		{
+			"2020-10-21",
+			args{
+				2020,
+				10,
+				21,
+			},
+			3,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := calWeek(tc.args.year, tc.args.month, tc.args.day)
+			if got != tc.want {
+				t.Errorf("calWeek = %d, want %d", got, tc.want)
+			}
+		})
+	}
+}
+
+func Test_isLeapYear(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  bool
+	}{
+		{
+			"1900",
+			1900,
+			false,
+		},
+		{
+			"2000",
+			2000,
+			true,
+		},
+		{
+			"2020",
+			2020,
+			true,
+		},
+		{
+			"2021",
+			2021,
+			false,
+		},
+		{
+			"1800",
+			1800,
+			false,
+		},
+		{
+			"1600",
+			1600,
+			true,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := isLeapYear(tc.input)
+			if got != tc.want {
+				t.Errorf("isLeapYear(%d) = %v, want %v", tc.input, got, tc.want)
+			}
+		})
+	}
+}
diff --git a/19_counting_sundays/main.go b/19_counting_sundays/main.go
new file mode 100644
index 0000000..b181fb5
--- /dev/null
+++ b/19_counting_sundays/main.go
@@ -0,0 +1,34 @@
+package main
+
+import "fmt"
+
+func main() {
+	sum := 0
+	for y := 1901; y <= 2000; y++ {
+		for m := 1; m <= 12; m++ {
+			if calWeek(y, m, 1) == 0 {
+				sum++
+			}
+		}
+	}
+	fmt.Println("sum:", sum)
+}
+
+func calWeek(y, m, d int) int {
+	//w=y+[y/4]+[c/4]-2c+[26(m+1)/10]+d-1
+	//
+	//公式中的符号含义如下，w：星期；c：世纪；y：年（两位数）；
+	// m：月（m大于等于3，小于等于14，即在蔡勒公式中，某年的1、2月要看作上一年的13、14月来计算，
+	// 比如2003年1月1日要看作2002年的13月1日来计算）；d：日；[ ]代表取整，即只要整数部分。
+	if m == 1 || m == 2 {
+		y--
+		m += 12
+	}
+	c := y / 100
+	y = y - c*100
+	return (y + y/4 + c/4 - 2*c + 26*(m+1)/10 + d - 1) % 7
+}
+
+func isLeapYear(year int) bool {
+	return year%400 == 0 || (year%4 == 0 && year%100 != 0)
+}