The Galactic Federation (Lucifer and his confederates) puts a heavy emphasis on symbols of light and energy. For this reason it is reasonable to expect them to publicly reveal themselves worldwide on a Sunday.

The day of the week known as “Sunday” has its roots in Greek and Roman paganism. The day was dedicated to the worship of the sun gods that pagans considered the source of all life and energy (names included Apollo, Apollyon, Phoebus, Helios, Sol Invictus, etc).
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The second most likely day of the week for the Arrival would be Wednesday. Wednesdays are associated with peak UFO activity and is also the day attributed to the demon Ashtaroth – also known as Commander Ashtar of the Ashtar Command.

Regardless of the exact day of the week the Arrival happens on, Sunday will eventually play an integral role in the Great Delusion.
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Government leaders and alien spokespersons from the Galactic Federation will address humanity and will proclaim a message of peace and prosperity. They will condemn war, poverty, corruption and environmental damage. Finally, they will likely elude to an existential threat to humanity that necessitated this sudden intervention.

With the approval and cooperation of global leaders, the greatest disinformation campaign will be launched against humanity.
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After the initial shock of the alien arrival has subsided slightly, and the narrative has been set, full disclosure will begin. These beings will come from a variety of different alien races that comprise membership in an extraterrestrial organization called the Galactic Federation (or something similar).

Andromedans, Arcturians and Pleiadians are just a few of the alien races that these beings might claim to be. Some of these beings will also include the apostate Sons of God, fallen angels that appear humanoid but possess superior abilities and wield technology so advanced that it may appear like magic, even to our modern society.