Learn Spanish (Spain): Se convocó una huelga para mañana.

Because the action is being done to a thing (una huelga) rather than a person. In Spanish, when a transitive verb with se acts on an inanimate noun, that noun becomes the grammatical subject. The verb agrees with it, which makes it the passive se. If it were plural, the verb would change to match: Se convocaron unas huelgas.

How would the grammar change if they called the workers instead of a strike?

If the object of the verb is a specific person or group of people, you use the impersonal se alongside the personal a. For example: Se convocó a los trabajadores. In this case, the verb always stays in the singular third-person (convocó) regardless of how many people were called, because the people act as the direct object, not the subject.

The strike is tomorrow, so why is the verb convocó in the past tense?

The verb convocar (to call or convene) refers to the action of announcing or scheduling the strike. That decision and announcement have already been completed, which is why it uses the preterite tense (convocó).

Why do we use para instead of por before mañana?

You use para when talking about deadlines, scheduled dates, or specific points in the future. Since the strike is an event scheduled for tomorrow, para is the correct choice here.