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You probably would have to use a cron job to do it, but I don't know how could we incorporate it into XOOPS.
Thinking here just loud - maybe you could have a block reading a content from a file, if one exist. And the cron job would rename a file on Thursday midnight to the correct name, and on Friday midnight rename to something else. If XOOPS doesn't see the file, it will be an empty block.
But this is just a thought. Maybe some of the gurus could come up with a more realistic suggestion