Hi Marcelo
Unfortunately, I haven't been able to find an easy way to group by week. In Report Designer there is no function for GetWeek or similar so dynamically grouping by week will be difficult. I know you can group a report using the GetMonth() function but that doesn't sound like it will be at a detailed enough level for what you require.
It might be possible to calculate a value for the week a work order belongs to using Calculated Fields in Report Designer but I don't know how you'd go about doing it.
It is possible in Method (with a fair amount of customization) to calculate a numeric value for the week a work order belongs to so you could possibly use something like that to assign a value to a custom field. Unfortunately, that won't be user friendly as it would have to be run manually before you run the report.
Jeff