Depends on what you require of this radical democracy and socialism.
If you´re fine with agrarian (theocratic) socialism with radical communal democracy (and a very weak overarching associative framework), then the 15th or 16th centuries might be doable. There were peasant revolts galore, and early radical reformation provided the theoretical framework, while communal assemblies, which had formed from the 12th century on throughout what is today Northwestern Spain, France, the Alps, Northern Italy, Southern Germany, the Netherlands, Belgium, the Czech Republic, Slovakia and Northwestern Romania, provided a solid political structure. Early modern absolutist monarchies with their relatively small professional armies were not yet an unsurmountable enemy for a revolutionary military force which uses the benefits of firearms and the military creativity of the many, as Jan Zizka`s Hussites have shown.
If you want something on the scale of a "nation" that builds on enlightenment philosophy and an industrial working class, then obviously you have to wait for and learn from the French Revolution. 1830 might be it, and of course 1848. Defending 1848 could have succeeded with a (paramilitarily) organised working class, so you`d need someone to prepare that. Some earlier Blanqui...