Learn Hardcore Japanese: kinyou ha isogasii desu.

は marks the topic: “As for Friday(s)…” So 金曜は忙しいです means “As for Friday(s), [I/it/someone] is busy,” often understood as “I’m busy on Friday(s).” It can also carry a mild contrast: Friday(s), as opposed to other days.

Where is the subject? Who is busy?

Japanese often omits the subject when it’s clear from context. Here it’s typically “I.” If you need to specify, you can add it: 私は金曜日は忙しいです (“As for me, as for Friday, I’m busy”). Context usually makes the subject clear.

Why not use に with the day (like “on Friday”)?

With events and actions, time is marked by に (e.g., 金曜日に行きます = “I’ll go on Friday”). But 忙しい is a state, so we normally topicalize the time with は: 金曜日は忙しいです. Saying 金曜日に忙しいです for “I’m busy on Friday” is unnatural.

What’s the difference between 金曜 and 金曜日?

Both mean “Friday.” 金曜日 (きんようび) is the full form. 金曜 (きんよう) is a common abbreviation, especially in schedules and headlines. In everyday speech people often say 金曜日; in writing (calendars, notes) 金曜 is very common. Either is fine here.

How do I pronounce the sentence, and why is read “wa”?