I guess this would need to be done right when the workbook opens, and
this was one of the things that I have tried, as described in my last
post in his topic, and that did not work.

Now in your reply, you wrote:
> "I notice that you've set the sheet  to "very hidden"."
I think you misread my post.
I wrote:
> For now, since I still need to see that sheet very often, I
did not use Dave's proposal for ThisWorkbook.Sheets("Sheet2").Visible
= xlSheetVeryHidden
>I guess I'll introduce that only later.

You also wrote:
>In fact, macros (VBA code) can access hidden sheets without trouble.
It's only when they try to do things that would normally cause the
sheet or its cells to come into view that you'll have trouble. I.e.
you can't select or activate cells on a hidden sheet,

But this is exactly what my macros do when they use the hidden sheet.
When a macro  need to update a webquery, it has to select the right
one, so I had to name the cell where the red "!" shows up indicating
that this is the beginning of the webquery that I want to update.

That is why I was so happy to be able to un hide  them but to
deactivate the screen updating,
Application.ScreenUpdating=False
because else the hiding  of the sheet would have been pretty useless:
without that part of code, I came back exactly to the situation that
I wanted to avoid.

All your replies brought me to try other things that did not work
well, but I will post  them separately, because for the
moment, without them everything seems to work well, and I still have
more urgent things in order to complete a first test workbook to send
to the site administrator.

One of those tasks being to replace by functions some cells where I
have so long formulas with tons of IF(... and AND( ; ;) that the
formula takes almost 3 lines, and I myself, when I look at them, I
can't remember why I wrote that, except that I know that they had to
be like that.
Now that I know how to write functions, I could easier document the
formulas with comments.