To determine the correct preposition to complete the sentence "I will complete all my assignments _ _ _ _ _ _ Saturday," we need to analyze the use of prepositions of time in English grammar.
Given that the sentence requires specifying a day ("Saturday"), the appropriate preposition to use is on.
Therefore, the complete sentence should be: "I will complete all my assignments on Saturday."
The correct answer is on.
This choice aligns with standard English grammar rules for using prepositions to indicate days of the week.