Learn Russian: Митинг был массовым, даже если мы не успели сделать плакаты. - The rally was massive, even if we didn't manage to make posters.

Даже если (even if) takes the indicative mood when the situation is factual rather than hypothetical. In this sentence, the speakers actually didn't have time to make posters, and the rally actually happened. If you used the subjunctive (даже если бы мы не успели), it would mean a hypothetical "even if we hadn't managed to..." which implies you actually did make them.

Why is массовым in the instrumental case here?

It acts as the predicate adjective after the past tense verb был (was). In Russian, when describing a state or characteristic with the past or future tense of быть, the instrumental case is usually preferred over the nominative. It emphasizes that this was the state of the rally at that specific time.

The English translation says "didn't manage to". Why use не успели instead of не смогли?

While both translate well to "didn't manage," успеть specifically means to have enough time to do something. Using не успели сделать tells us that the deadline (the start of the rally) arrived before they could finish. Не смогли would just mean they were physically or circumstantially unable to do it.

Is митинг used for any kind of meeting?