Mechanism of the Aldol reaction:

The mechanism starts with the creation of the enolate ion. It is significant to realize that not all of the acetaldehyde is transformed to the enolate ion and thus we still comprise molecules of acetaldehyde present in similar solution like the enolate ions. Because acetaldehyde is susceptible to nucleophilic attack, next stage in the mechanism is the nucleophilic attack of the enolate ion upon acetaldehyde.

Figure: Mechanism of the Aldol reaction.

The enolate ion comprises two nucleophilic centers - the carbon and the oxygen - although the preferred reaction is at the carbon atom. The 1^st step is nucleophilic addition of the aldehyde to make a charged intermediate. The second step includes protonation of the charged oxygen. Because a dilute solution of sodium hydroxide is employed in this reaction, water is obtainable to supply the essential proton.

Note: it would be wrong to depict a free proton (H⁺) because the solution is alkaline.