You don't need to book your spot for Särkänniemi
In case of Fixed Date Särkänniemi Pass, the time of visit is selected at the time of purchase. You can change the date of visit to another date with an additional fee in case there is space on the desired date. There is a personal link in the order confirmation which you can access to change the date. The pricing is dynamic so the prices go up according to demand.
There is a specific daily capacity reserved for Season Pass holders, so you do not need to book your visit in advance if you are visiting with a Season Pass.