The starter keystroke for fill in series is alt+h, then f, i, s. You have to select the cells you want to fill, including the one that starts the series. the cells other than the first one must be blank.
Tabbing through the resultant dialog box:
step value is 1 (or however many days you want).
columns, not rows.
where it says linear, arrow down to date.
The next box says day, week, etc.
Once you have chosen all those things, tab to okay and you're in business.