Hi Abhijit, in the updated Scrum Guide from 2013, there is only one Sprint Planning meeting which is 8 hours for a 4-week sprint. The reason is that some teams, (instead of taking the PBIs into the Sprint Backlog first and breaking them down afterwards as in the previous version) take the first PBI, break it down, take the next one etc. until they think they have enough. The advantage is that they gain a deeper understanding of the work to be done, so they can make a better estimate how much they can get done. The team decides this, because it is self organizing.
An important thing to bear in mind is the crafting of the Sprint Goal. Suppose you spent time breaking down each selected PBI before deciding on the Goal. Would that be more or less efficient than identifying a suitable goal prior to this decomposition? Either situation might be the case.