i use datamerge to make planner diaries and the OP has answered their own question in their post: “set up my excel file differently”.
start by making a new indesign file with two pages (not facing pages but two pages as a readers spread by turning “allow document pages to shuffle” off). set this “master spread” up as desired, leaving positionals for the variable data to fit into.
the real magic happens in excel. the first row is the header row, and this is where titles such as WEEK, MONDAY, TUESDAY, etc would go. the reason that i give each day its own data field is that indesign DOESN'T have what MS word has in merging: the “next record” feature, which would make life a lot simpler, so this is the workaround. i don't agree that this would be difficult provided that sums and formulii were used to create the dates, rather than being keyed in manually.
the second row is then the guts of the data, and because excel handles dates really well, one only has to enter 01/01/11 for example to get a date to appear in a cell. from there, making the date increase is a matter of using sums in the adjacent cells. also, the cells can be formatted to be as they appear in the example, or read such as 1 Jan 2011; Saturday January 1 2011, etc… and this is all done by using the format cells feature of excel.
ultimately, when all the data for the year has been created, export it as a txt file rather than a csv and then use the datamerge feature of indesign to put it all together.
i must add though, if the “next record” feature existed in indesign, datamerge would be so much better and would allow for the construction of catalogues, mailing labels (not that software of this pricetag should be delegated to such a rubbish task), etc WITHOUT having to use a third party plug-in or brush up on XML.
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