This is a very tricky question and asked in certification exam as well as in many interviews and you would find a lot of discussion about “Concept of Sprint Zero“. Many management and software leaders try to demonstrated that they know the Agile SCRUM concept and from the day one they would like to run everything under sprint (a time-box period) but they would not delivery even single feature which is releasable and that’s why they name it zero to give a feeling to rest of the stake holder that don’t expect anything out of this sprint as it is zero sprint but it is a sprint. Look at the question asked in SCRUM Open assessment and it clearly says that there is no sprint-0 in Agile SCRUM.
Sprint Zero or Preparation Phase before we start Sprint -1
The heart of Scrum is a Sprint, a time-box of one month or less during which a “Done”, usable, and potentially releasable product Increment is created. A sprint therefore must have a sprint goal where something valuable to the business is delivered, however small that may initially be. The initial sprint will likely contain much more grooming activity to prepare a backlog of Product Backlog Items (PBI) than future sprints. What is important is to ask what can initially be done, and then begin working on that item.
Again, if we’re being asked to treat the first sprint like an actual sprint (per Scrum Guide), then why refer to it as “Sprint 0”? Just call it Sprint 1, and get started!
If we see project management life cycle (PMLC), scrum falls in more into execution phase. Scrum does not address project initiation or planning efforts. Many Agile gurus now talk about creation of vision document and road map as precondition before starting scrum. Scrum also consider “highly motivated team” is available at the beginning.
When Can I start a Sprint?
Before organisation or business function start a sprint, there must be an organisation level process which includes crafting and sharing a vision, initial Product Backlog creation, Scrum Team formation and familiarization, and environments (development, build, testing, staging, source control, tools, etc.), initial version of Definition of Ready (DoR), definition of Done (DoD). An organisation which has been developing software using Agile SCRUM way must have these processed well defined and we can name it as preparatory phase, however such phase can not be labelled as Sprint Zero.
How much backlog grooming is needed before we start a Sprint?
During the preparation phase and based on nature of project, team or governing body can decide what would be the length of each sprint (a month or less than that) and sufficient product backlog item grooming is necessary so all the resources are utilised during the execution. In theory, you only need enough work on a Product Backlog to start one true Sprint. The rest of the backlog can be refined on a just-in-time basis. However, this increases certain risks (such as a failure to prepare enough work for the next Sprint Planning session), and so it is a wise step to have at least groom two or three sprints’ worth of work and make sure they meet definition of ready criteria (DoR).