Challenge 3: Ctrl + Drag on a Weekday Series — Solution Walkthrough

1. A1 = "Monday". Drag its fill handle across to G1 (no Ctrl held).
2. A1 = "Monday" again. This time hold Ctrl while dragging across
   to G1.

Expected result:

Without Ctrl (plain drag):
  Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday

With Ctrl held while dragging:
  Monday, Monday, Monday, Monday, Monday, Monday, Monday

Explanation:

"Monday" is a value Excel recognises as belonging to a built-in fill
series (the 7-day weekday cycle), so its DEFAULT drag behaviour is to
continue that series automatically — exactly the same category of
built-in recognition covered in this chapter for month names and
recognisable dates.

Holding Ctrl toggles the default behaviour to its opposite. For a
single plain number, the default is "copy unchanged," so Ctrl flips
it to "start a +1 series." For a recognised series value like a
weekday name, the default is "continue the series," so Ctrl flips it
the other way — to "just copy this exact value, don't advance
through the cycle."

In both cases, Ctrl means the same underlying thing: "do the OPPOSITE
of whatever this value would normally do when dragged." It's not that
Ctrl always means "make a series" — it means "invert today's default,"
and the default itself depends entirely on whether Excel recognises
the dragged value as belonging to a known series in the first place.

Notes:
- This single "Ctrl inverts the default" rule covers both this
  exercise and Exercise 1's plain-number case — it's worth learning
  as one general rule rather than two separate special cases.