To elaborate , check out "Conditional Formatting" in
Excel Help. It's on the Format menu.



In your case, you'd go to cell  A1, hit Format/Conditional Formatting.,
select "Formula Is" (instead of "Cell Is") and enter =DAY(TODAY())=1, select
Format, go to the Patterns tab, and select Yellow as the background  color,
hit OK and OK.



Then go to B1, and do the same thing with "Formula Is" set to
=DAY(TODAY())=2, with Yellow pattern.

Go to A22, and do the same thing with "Formula Is" set to =DAY(TODAY())=3,
with Yellow pattern.

And go to C17, and do the same thing with "Formula Is" set to
=DAY(TODAY())=4, with Yellow pattern.

Etc.



Does this make sense? Pretty cool feature.