This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
Okay... does that mean that the actual XML document has 35 <day> elements, and you want to select the ones with number="7"? Or does it mean that you need to convert the day of the month to the day of the week? Or does it mean something else? I think you should clarify your requirements before you start implementing them.