Numbers- - How to Autofill dates in a row in Numbers

Grab the little yellow bubble on either side of the cell and drag away from the cell. It will auto-fill the adjacent cells.

For a date you can drag from a single cell and it will automatically add one day to the next cell.

For other series, like 1-3-5... or A-B-C..., you have to select at least two cells to begin with so that Numbers can deduct the increment.