NOTE: While Iran keeps repeating that it will respond (and it will), it is much harder for Israel, the U.S. and their allies to predict exactly when.
Last time, it was about sending a message. It was a show of force. Iran knew that the 400-500 explosive-laden drones would take many hours to arrive in Israel, and that they would almost all be intercepted on the way to their targets. This also showed the traitors among the Arab nations. It was Iran's first direct attack on Israel and they did not know how Israel would respond.
The point was to show that it would take the entire U.S. Air Force, U.S. Navy, RAF, French Air Force, Jordanian Air Force & the Israeli Air Force to intercept them. As I said, it was a show of force. The missiles also largely consisted of older variants, used as decoys, while the small amount of 'modern' missiles such as the 'Kheybar Shekan' all hit their targets, although purposely avoiding massive damage to the Israeli airbases and striking the runways instead.
This time, Iran likely will not use drones or even cruise missiles. It might be an attack consisting solely of ballistic missiles, which take only 12 minutes to reach Israel and require much less staging / preparation.
Not only is it much more difficult for the United States to know about the exact timing of the attack because Iran refuses to communicate, it will also take U.S. intelligence way longer to detect the signs of an imminent attack; Iran's ballistic missiles can be launched from silos and mobile launch platforms within minutes.
Additionally, some people say that Iran spends too much time telegraphing its response. Iran does this because it doesn't just want to restore deterrence, it wants the world to see.
On April 13th, it took 5 nations to intercept Iran's extremely 'tame' attack; and even then, all of the Iranian advanced missiles got through the defenses & hit their targets.
It doesn't matter how long Israel prepares, Iranian missiles are capable of penentrating defenses as demonstrated on April 13. It's a matter of intent, not capability. People who think it makes a difference whether Iran attacks now or in one week; it doesn't.
If Iran aims to cause casualties and damage, it will. Especially if we see large scale use of advanced missiles such as the Kheybar Shekan & Kheybar Shekan-2 (hypersonic), Khorramshahr-4 and Fattah (hypersonic).