Luxemburg is completely correct to say that the outbreak of a proletarian revolution cannot be "predetermined" by the calendar, and nothing to the contrary will ever be found in Lenin. Lenin, like Luxemburg, was convinced that these elemental mass explosions, without which a revolution is unthinkable, can neither be "organised" according to rules nor "commanded" by a row of disciplined non-commissioned officers. Lenin, like Luxemburg, was convinced of the mighty arsenal of creative energy, resourcefulness and initiative that a truly broad mass action unfurls and will always unfurl.
The difference between the Leninist theory of organisation and the so-called theory of spontaneity - which can be attributed to Luxemburg only with important reservations - is thus to be found not in an underestimation of mass initiative but in an understanding of its limitations. Mass initiative is capable of many magnificent accomplishments. But by itself it is not able to draft, in the course of the struggle, a complete, comprehensive program for a socialist revolution touching upon all social questions (not to mention socialist reconstruction); nor is it alone capable of bringing about a sufficient centralisation of forces to make possible the downfall of a centralised state power with its repressive apparatus resting on a full utilisation of the advantages of its "inside lines" of communication. In other words, the limitations of mass spontaneity begin with the understanding that a victorious socialist revolution cannot be improvised. And "pure" mass spontaneity always boils down to improvisation.
What is more, "pure" spontaneity exists only in books containing fairy tales about the workers movement - but not in its real history. What is understood by "spontaneity of the masses" are movements that have not been planned out in detail ahead of time by some central authority. What is not to be understood by "spontaneity of the masses" are movements that take place without "political influence from the outside." Scratch off the blue coat of an ostensibly "spontaneous movement" and you will find the unmistakable residue of a bright red veneer. Here a member of a "vanguard" group who set off a "spontaneous" strike. There a former member of another "left-deviationist" affiliation, who has long since left it but who received sufficient mental equipment to be able, in an explosive situation, to react with lightning speed while the anonymous mass was still hesitating.
In one case, we will be able to detect in "spontaneous" action the fruits of years of "underground activity" by a trade-union opposition, or a rank-and-file group; in another case, the result of contacts that, for a rather long period of time, have patiently - and without apparent success - been nurtured by shop colleagues in a neighbouring city (or a neighbouring factory) where the "left- wingers" are stronger. In the class struggle too there is no such thing as a goose "spontaneously" falling from heaven already cooked.
Thus, what differentiates "spontaneous" actions from the "intervention of the vanguard," is not at all that in the former everyone in the struggle has reached the same level of consciousness, whereas in the latter "the vanguard" is distinct from "the mass." What differentiates the two forms of action is also not that in "spontaneous" actions no solutions have been carried into the proletariat from "outside," while an organised vanguard relates to the elementary demands of the mass "in an elitist fashion," "imposing" a program upon it. Never have there been "spontaneous" actions without some kind of influence from vanguard elements. The difference between "spontaneous" actions and those in which "the revolutionary vanguard intervenes" is essentially that in "spontaneous" actions the nature of the intervention of the vanguard elements is unorganised, improvised, intermittent and unplanned (occurring by chance in this plant, that district, or that city), while the existence of a revolutionary organisation makes it possible to co-ordinate, plan, consciously synchronise, and continuously shape this intervention of the vanguard elements in the "spontaneous" mass struggle. Nearly all the requirements of Leninist "supercentralism" are based on this and this alone.
Only an incorrigible fatalist (i.e., a mechanical determinist) could be convinced that all mass explosions had to take place on a given day just because they broke out on that day, and that, conversely, in all cases where mass explosions did not occur it was because they were not possible. Such a fatalistic attitude (common to the Kautsky-Bauer school of thought) is in reality a caricature of the Leninist theory of organisation. In any case, it is characteristic that many opponents of Leninism, who in opposing Lenin have so much to say about "mass spontaneity," at the same time fall into this vulgar, mechanical determinism without realising how much it contradicts their "high esteem" for "mass spontaneity."
If, on the other hand, one proceeds from the inevitability of periodic spontaneous mass explosions (which occur when socio-economic contradictions have ripened to the point where the capitalist mode of production in fact has to periodically produce such prerevolutionary crises), then one has to understand that it is impossible to determine the exact moment when this will happen since thousands of minor incidents, partial conflicts and accidental occurrences could play an important role in determining it. For this reason, a revolutionary vanguard which at decisive moments is able to concentrate its own forces on the "weakest link," is incomparably more effective than the diffuse performance of large numbers of advanced workers who lack this ability to concentrate their forces. 
The two greatest workers struggles to take place in the West - the French May 1968 and the Italian fall 1969 - entirely confirmed these views. Both began with "spontaneous" struggles prepared neither by the trade unions nor by the big social-democratic or "communist" parties. In both cases individual, radical workers and students or revolutionary nuclei played a decisive role in here or there triggering a first explosion and providing the working masses with the opportunity to learn from an "exemplary experience." In both cases millions upon millions came into the struggle - up to ten million wage earners in France, up to fifteen million in Italy. This is more than ever before seen - even during the greatest class struggles following the first world war.
In both cases the spontaneous tendency demonstrated by the workers went way beyond the "economism" of a purely economic strike. In France this was attested to by the factory occupations and numerous partial initiatives, in Italy not only by huge street demonstrations and the raising of political demands, but also by the embryonic manifestation of a tendency toward self-organisation at the point of production, i.e., by the attempt to take the first step toward establishing dual power: the election of delegati di reparto. (In this sense, the vanguard of the Italian working class was more advanced than the French, and it drew the first important historical lessons from the French May. ) But in neither case did these powerful, spontaneous mass actions succeed in overthrowing the bourgeois state apparatus and the capitalist mode of production, or even in advancing a mass understanding of the objectives that would have made such an overthrow possible within a short period of time.
To recall Trotsky’s metaphor from The History of the Russian Revolution: the powerful steam evaporated for lack of a piston that would have compressed it at the decisive moment.  Certainly, in the final analysis, the driving force is the steam, i.e., the energy of mass mobilisation and mass struggle, and not the piston itself. Without this steam the piston remains a hollow shell. Yet without this piston even the most intense steam is wasted and accomplishes nothing. This is the quintessence of the Leninist theory of organisation.