org-timeline-show-empty-dates

org-timeline-show-empty-dates is a variable defined in `org-agenda.el'.
Its value is

3

Documentation:
Non-nil means `org-timeline' also shows dates without an entry.
When nil, only the days which actually have entries are shown.
When t, all days between the first and the last date are shown.
When an integer, show also empty dates, but if there is a gap of more than
N days, just insert a special line indicating the size of the gap.

You can customize this variable.