The small gathering at a location to be confirmed in London - and at which Mr Trump is not expected to attend - is likely to be easily outnumbered by protesters involved in a series of demonstrations organised for the coming days, including the flying of a 20ft-high Trump inflatable near Parliament.
But any good vibrations from that moment soon dissipated when Trump, the same day, announced plans to ban migrants from seven Muslim-majority countries - a decision that drew fierce global criticism and appeared to blindside May.
Trump will travel by helicopter once there, making it unlikely he'll see protests firsthand.
Mr Trump is believed to be spending the weekend playing golf at his Turnberry resort in South Ayrshire, which he bought in 2014, although there has been no official confirmation of his plans.
According to a notice posted on the embassy's website on Tuesday, "Numerous demonstrations are being planned for July 12 to 14, 2018, surrounding" Trump's visit.
The president is scheduled to meet with British Prime Minister Theresa May and Queen Elizabeth during his visit. We talk about Trump and Angela Merkel because it's "difficult" " said Leslie Vinjamuri, head of the USA and Americas program at the Chatham House think tank.
"We are at a very key point in history right now, and we need to take a stand".
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A statement of the U.S. embassy website warned American citizens to "be aware your surroundings", "exercise caution if unexpectedly in the vicinity of large gatherings that may become violent" and "monitor local media for updates".
Last week, Downing Street revealed an itinerary that appeared to keep the US leader out of Britain's capital, which has raised suspicions that May is trying to shield Trump from widespread protests planned over the visit.
"I have North Atlantic Treaty Organisation, I have the United Kingdom, which is in somewhat turmoil, and I have (Vladimir Putin)", Trump said as he set off on his trip to Europe which includes a meeting with the Russian President in the Finnish capital Helsinki.
As May and Trump hold talks throughout the week, Philip will be expected to entertain Melania during the visit.
Assistant Chief Constable Chris Shead, from the National Police Coordination Centre, said: "Police forces are working together on a significant, multi-faceted security operation supporting the presidential visit to the UK".