Thanks alot Walaa
I have wrote the following code and works fine.
EntityField dayField = new EntityField("Day", new DbFunctionCall("To_Char", new object[] { CustomerFields.BirthDate, "DD" }));
EntityField monthField = new EntityField("Month", new DbFunctionCall("To_Char", new object[] { CustomerFields.BirthDate, "MM" }));
filter.Add(dayField == BirthDateTimePicker.Value.Day);
filter.Add(monthField == BirthDateTimePicker.Value.Month);