The earlier May bank holiday and a later half term saw passengers using Heathrow last month decline by 0.8% to 6.2 million.
However, the west London hub, which is battling with rival Gatwick to win government approval for expansion, reported 1% growth in passenger levels in the year to date.
This was driven by larger, quieter aircraft using the capacity-constrained airport, it said.
Within emerging markets, China traffic grew by 7.5%, Mexico by 3.7% and East Asia by 2.5%, due in part to increased British Airways services to Kuala Lumpur and new a Garuda Indonesia service following its switch from Gatwick.
Chief executive John Holland-Kaye said: “Over the last 70 years Heathrow, Britain’s biggest port, has played a pivotal role in helping Britain thrive as an island trading nation.
“Expanding Heathrow will mean 40 new direct connections across the globe, exporting more goods to more markets for the next 70 years and beyond.
“Heathrow is the right choice to secure a stronger economic future for all of the UK.”
His comments came a day after transport secretary Patrick McLoughlin suggested that a decision on a third runway at Heathrow could be further delayed until September.