Add `getWeekday` function

src/main.zig

@@ -228,6 +228,16 @@ test "new timezone utc" {
     try testing.expectEqual(@as(TimeZoneTag, tz1), .utc);
 }
 
+pub const WeekDay = enum(u3) {
+    thursday = 0,
+    friday,
+    saturday,
+    sunday,
+    monday,
+    tuesday,
+    wednesday,
+};
+
 pub const DateTime = struct {
     year: Year,
     month: Month,
@@ -283,6 +293,12 @@ pub const DateTime = struct {
         if (self.getOffset()) |ofs| seconds -= ofs.asSeconds();
         return seconds;
     }
+
+    pub fn weekday(self: Self) WeekDay {
+        const ts = self.toTimestamp();
+        const days = @divTrunc(ts, SECONDS_PER_DAY);
+        return @intToEnum(WeekDay, @rem(days, 7));
+    }
 };
 
 test "get year" {
@@ -336,3 +352,16 @@ test "to timestamp negative offset" {
     };
     try testing.expectEqual(dt.toTimestamp(), 1686633682);
 }
+
+test "get weekday" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 13,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    try testing.expectEqual(dt.weekday(), .tuesday);
+}