To clarify this point, it should be mentioned that there is a Sunday bar in the downloadable data source. It is only one hour long, and this data is counted as part of the previous week for purposes of creating the weekly bar.
This could be an issue for indicators and strategies that work from the daily timeframe, since the one hour of data for Sunday would be given as much significance as any other day containing 24 times as much price action.
Tantalus Research - Developing 21st Century Trading Systems.
Yes, Tantalus is right, but you can remove Sunday bar by shifting data ("Shift time" parameter in data center). Or you can use broker-specific data from our Data Service (some brokers work in GMT time zone and thay do not have Sunday bar).
... tell us EXACTLY what the Sunday data represents.
I can only imagine two things: the early morning trade in Auckland and Sydney which is still Sunday in London or GMT time.
Most of the forex companies I know "close the daily bars" at NY closing.
But Friday New York closing is Friday 22.00 PM London time. So Friday trading can never be the explanation.
Or maybe the weekend trading moves? (do not believe that myself).
So in my view the "Sunday data" should not be added to the PREVIOUS week but to the NEXT week, unless there is another explanation for the Sunday data.
Why this is not details but important: everything we do is based on the data we download. If you do not know exactly what data you are using then you do not know at what results you are looking. Most of the time the "Sunday data" will have no big changes. But with a financial crisis in Europe, a possible defaulting in the US, an earthquake and a nuclear disaster in Japan, all in 6 months time, I would never be so sure it will be always like that.