Spot block vs reserved instances
Anything that "needs" to happen in a given time frame is not a candidate for spot instances, including spot blocks.

Spot capacity is, by definition, surplus/idle capacity, subject to availability, so your spot block may simply fail to start, as capacity may not be available.  It is less expensive for exactly that reason -- AWS is selling surplus capacity for which no one is currently willing to pay more.

Information about the pool of compute capacity that is currently flagged internally as eligible for participation in the spot market is not publicly accessible, and although it was possible to make some reasonable assumptions in the past about capacity based on signals you could obtain from the real-time pricing, the spot pricing algorithm was modified in 2017 in a way that largely masks those signals (and the change, in my opinion, was a good thing for any customer who was not aggressively gaming the system).

Spot blocks are intended to run for a committed period of time, so they are less likely to be interrupted than other spot instances, but they can initially start up successfully and then subsequently be interrupted at any time.


  In rare situations, Spot blocks may be interrupted due to Amazon EC2 capacity needs. In these cases, we provide a two-minute warning before we terminate an instance, and you are not charged for the terminated instances even if you used them.
  
  https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-requests.html#fixed-duration-spot-instances