Not that easy to answer as DB ticketing is a … well that wasn't your question, so lets stop here

The most expencive fare "Normal Ticket" can be booked at any time and there is no need to buy in advance. Some discounted tickets such as "Plan & Spar 50 and 25" have to buy at least 3 days in advance and this fares have a restricted inventory. Plan & Spar will not work for you anyway as this fare requires a minimum stay etc. etc.

"Freitzeit Ticket" and "Schönes Wochenende Ticket" can be bought at any time before travel so even at the time of departure. But if you won't use your ticket there won't be any refund.

If you want to buy the Freizeit Ticket or a Schönes Wochenende Ticket anyway you can easiliy do so at the railway station. Shouldn't be any problem.