XML Tutorial/XML Schema/recurringDay

For an event that occurs on the same day of each month, use xsd:recurringDay

File: Schema.xsd
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">
  <xs:element name="payday" type="xs:recurringDay" />

</xs:schema>

File: Data.xml
<?xml version="1.0"?>
<payday>---30</payday>