Learn Hardcore Japanese: syuumatu ni ibento ni sankasimasu. - I will participate in an event on the weekend.

They serve different roles. The first に marks a specific point in time (i.e. “on the weekend”). The second に marks the target of the intransitive verb 参加します (“to participate in / attend”). So literally: “On the weekend, I participate in an event.”

Shouldn’t we use the object marker を with 参加します (e.g. イベントを参加します)?

No—参加する is considered an intransitive verb in Japanese, so it takes に to indicate what you join or attend. Using を here would be ungrammatical. You always say イベントに参加します, not イベントを参加します.

Why is there no explicit subject like “I” in this sentence?

Japanese often omits the subject when it’s clear from context. Here it’s assumed you’re talking about yourself. If you want to be explicit or contrast, you can add 私は at the beginning:
私は週末にイベントに参加します。

What’s the “normal” word order in Japanese? Could I say イベントに週末に参加します?

Japanese word order is fairly flexible, but the typical hierarchy is:

Topic/time expressions
Location/target expressions
Verb phrases
Hence 週末にイベントに参加します is the most natural. Saying イベントに週末に参加します is grammatically possible but sounds awkward—time usually comes first.

Can I drop the first に (after 週末) in casual speech?

Dropping time particles like に is generally uncommon and may sound incomplete. In very casual chat you might hear 週末イベントに参加する (dropping both に and ます), but that’s more slangy. For clear communication it’s best to keep 週末に.

What’s the difference between イベントに行きます and イベントに参加します?

行きます simply means “go,” focusing on movement: “I’ll go to the event.”
参加します means “participate” or “attend,” emphasizing joining in the activities. Use 参加します when you want to stress your involvement.

How do I say “I’m planning to attend an event this weekend”?

Add 予定 (“plan”) after the dictionary form:
週末にイベントに参加する予定です。
This literally means “I have plans to participate in an event on the weekend.”

How can I make this sentence more casual or more polite?

Casual: drop ます and use the dictionary form—週末にイベントに参加するよ。
Polite/honorific: you can soften it further—週末にイベントに参加させていただきます。

What’s the difference between using は and に after 週末 (e.g. 週末はイベントに参加します)?

週末に marks “when” you do the action.
週末は makes “weekend” the topic, implying you’ll talk about what happens on weekends in general.

週末にイベントに参加します。 “I will attend an event this weekend.”
週末はイベントに参加します。 “As for weekends, I attend events (perhaps regularly or as a habit).”