The background to the United States and Israel setting the morning of the 28th (local time) as the timing for an attack on Iran appears to have been a decision that took into account the meeting time of Iran's top leadership.
Local outlets including The Wall Street Journal (WSJ) and The New York Times (NYT) reported that the United States and Israel set the launch time of the airstrikes based on intelligence about the meeting schedule of senior Iranian officials.
While defenses can be vulnerable late at night or at dawn, they instead identified the time and place when senior Iranian officials would gather in one location, increasing the efficiency of the attack to the level of annihilation.
An Israeli military official said multiple people were killed when three locations where senior Iranian officials were gathered were attacked simultaneously that day. A person familiar with the operation also said one of the surprise tactics in this operation was that U.S. forces, after bolstering deployments in waters around Iran for weeks, carried out the strike in broad daylight.
In fact, the United States had deployed its largest firepower around Iran since the start of the Iraq War in 2003. On the day of the attack, an aircraft carrier was in nearby waters off Iran, and destroyers and littoral combat ships were on standby armed with ship-to-ground missiles.
Missiles launched from there flew to the designated coordinates—such as the locations where Iranian officials had gathered and Khamenei's residence—once the order to commence the attack was given. Hundreds of drones were also mobilized for this attack.
The timing chosen by the United States and Israel in fact proved effective in boosting the impact of the airstrikes. Reuters reported that Iran defense minister Aziz Nasirzadeh and Islamic Revolutionary Guard Corps (IRGC) commander in chief Mohammad Pakpour were believed to have been killed in the strikes.
Not long after reports emerged of the deaths of senior Iranian military figures, major U.S. and Israeli media outlets poured out reports speculating that Khamenei had died. The outlet said the Israeli military dropped about 30 bombs on Khamenei's residence, and at the time Khamenei was said to have been underground.