diff --git a/src/testcases/org/apache/poi/ss/formula/functions/WeekdayFuncTest.java b/src/testcases/org/apache/poi/ss/formula/functions/WeekdayFuncTest.java
new file mode 100644
index 0000000..3e0df8c
--- /dev/null
+++ b/src/testcases/org/apache/poi/ss/formula/functions/WeekdayFuncTest.java
@@ -0,0 +1,54 @@
+package org.apache.poi.ss.formula.functions;
+
+import org.apache.poi.ss.formula.eval.*;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+
+public class WeekdayFuncTest {
+    /**
+     * der 1. Januar 2008 die fortlaufende Zahl 39.448, da dieser Tag nach 39.448 Tagen
+     * auf den 01.01.1900 folgt.
+     */
+
+    @Test
+    public void testEvaluate() throws Exception {
+        assertEquals(2.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(2.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(1.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(1.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(2.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(0.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(3.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(1.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(11.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(7.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(12.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(6.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(13.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(5.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(14.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(4.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(15.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(3.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(16.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(2.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(17.0)}, 0, 0)).getNumberValue(), 0.001);
+
+        assertEquals(3.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(3.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(1.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(2.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(2.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(1.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(3.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(2.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(11.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(1.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(12.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(7.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(13.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(6.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(14.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(5.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(15.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(4.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(16.0)}, 0, 0)).getNumberValue(), 0.001);
+        assertEquals(3.0, ((NumberEval)WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(39448.0), new NumberEval(17.0)}, 0, 0)).getNumberValue(), 0.001);
+    }
+
+    @Test
+    public void testEvaluateInvalid() throws Exception {
+        assertEquals(ErrorEval.VALUE_INVALID, WeekdayFunc.instance.evaluate(new ValueEval[]{}, 0, 0));
+        assertEquals(ErrorEval.VALUE_INVALID, WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(1.0), new NumberEval(1.0)}, 0, 0));
+
+        assertEquals(ErrorEval.NUM_ERROR, WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(-1.0)}, 0, 0));
+        assertEquals(ErrorEval.VALUE_INVALID, WeekdayFunc.instance.evaluate(new ValueEval[]{new StringEval("")}, 0, 0));
+        assertEquals(ErrorEval.VALUE_INVALID, WeekdayFunc.instance.evaluate(new ValueEval[]{new StringEval("1"), new StringEval("")}, 0, 0));
+        assertEquals(ErrorEval.NUM_ERROR, WeekdayFunc.instance.evaluate(new ValueEval[]{new StringEval("2"), BlankEval.instance}, 0, 0));
+        assertEquals(ErrorEval.NUM_ERROR, WeekdayFunc.instance.evaluate(new ValueEval[]{new StringEval("3"), MissingArgEval.instance}, 0, 0));
+        assertEquals(ErrorEval.NUM_ERROR, WeekdayFunc.instance.evaluate(new ValueEval[]{new NumberEval(1.0), new NumberEval(18.0)}, 0, 0));
+    }
+}
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