On June 6, 1944, Allied troops landed in Normandy, France, as what is now known as the D-Day invasion of World War II. Codenamed Operation Overlord, the invasion was a key victory for the Allies; at the time, France was occupied by Germany, and this was the first step toward liberating France and opening up a Western front in Europe against the Nazis. Planning the invasion took months, as every detail -- even those laypeople would typically overlook -- was considered and addressed. 

But even the best-laid plans often go awry, and D-Day was no different. Just a day before the campaign was supposed to begin, a 21-year-old woman from Ireland gave the Allies some information that put the plans on the shelf and probably saved the invasion from turning into a failure. And she didn't even know what she had done.

The Allied invasion was supposed to occur on the morning of June 5, 1944, a date specifically chosen because of the moon. The Allies wanted to invade under a full moon, as that would give their recon pilots maximum light for their scouting flights the night before, and also create the highest tides on the beach and therefore lessen the amount of time that landing parties were exposed to enemy fire. That gave the Allies a window of only a few days each month, with June 5-7 being the optimal range that month. General Dwight Eisenhower, the Supreme Commander of the Allied Forces, chose the 5th to give him some flexibility if things went wrong -- and that proved smart because things definitely went wrong.

Ireland was officially neutral in World War II but it had a longstanding agreement with the UK around weather reports. In an era before Doppler radar and weather satellites, you needed people near emerging patterns to tell you what was happening. And Ireland was well-situated to help. The Blacksod Lighthouse in County Mayo off the northeastern shores of Ireland had a weather station. (Here's a map showing where the lighthouse is.) And as the Independent (Ireland) reports, "on June 3, 1944, the night of her 21st birthday, Maureen [Flavin] Sweeney, was taking pressure and temperature readings at Blacksod weather station on Ireland's west coast as part of her duties as a post office assistant." And at 1 AM, she noticed something was amiss -- the barometric pressure had dropped. She relayed the information to Met Éireann, the Irish meteorological service, and went on her way. But as morning approached, she received a strange phone call. As the Irish Times reported, "an English woman" was on the other line, "asking that she 'please check… please repeat' the report." So, with her future husband Ted, she checked and rechecked the readings every hour, and, per the Irish Times, "confirmed that a storm would indeed hit the English Channel on June 5th."

The Allies, in large part because of Sweeney's report, delayed the invasion by a day. And it's a good thing they did. At 3:30 AM on the 5th, Eisenhower, stationed across the English Channel from France, observed "shaking and shuddering under a wind of almost hurricane proportions" per his own writing. When the Allies invaded the next day, more than 4,000 Allied lives were lost, and that number would have likely been much higher but for the decision delay. Sweeney's weather reports had saved many, many lives.

And incredibly, she had no idea. She and Ted thought they were just sharing the weather with their community and country. The D-Day invasion was a secret and Ireland was, officially, neutral, so telling a random family about their efforts would have made no sense at the time -- and then as time moved on, history almost forgot about their efforts. As Irish American recounts, "the Sweeney family, Ted, his mother Margaret and sister Frances, together with Maureen, had been reporting on the hour, twenty-four hours a day, to the Meteorological Service in Dublin for the length of World War II. This hourly reporting continued until an automatic meteorological station was brought into operation [in 1956]. Only then, in 1956, did the now husband and wife couple, Ted and Maureen, learn about the history-changing events that their met reports had contributed to in 1944."

Ultimately, though, Maureen Sweeney's contributions earned her some recognition. On June 20, 2021, as seen here, the U.S. House of Representatives gave the 98-year-old Sweeney a special honor for her "skill and poise as the weather forecaster [ . . . ] saved countless Allied lives and ensured the success of the D-Day invasion."