Your goals in the opening are to develop your pieces, capture the center, and get your King into safety. You have seven pieces to develop : four minor pieces, two Rooks, and the Queen.
The Rooks can't be developed until the back rank is cleared. This means that you generally develop them after the minor pieces and the Queen.
The Queen shouldn't be developed until you have an idea which square is the most suitable. This depends on the disposition of your opponent's forces and it takes time to discover where they are going to be developed.
Conclusion? The minor pieces should be developed first. Since you have to move two Pawns to develop the Bishops, it will take at least six moves to develop the minor pieces. After those six moves you should also have a good idea what your opponent is doing.
The four minor pieces include two Bishops and two Knights. You have several possible deployments for the Bishops -- (1) long diagonal or fianchetto plus (2) which square on the chosen diagonal -- totaling seven squares each from their initial positions. Since you have fewer possibilities for the Knights -- three squares each (one of which is dubious) -- you will usually have a better idea where your Knights should be developed.
This is why you often hear, 'Develop Knights before Bishops'. You will be able to determine the best squares for the Bishops after a few moves have been played.
