Weather: If it rains on Saint Medard's day, there will be rain for forty days after, unless Saint Barnabas' day is dry, or (as I have also heard it) it will rain until at least St. Barnabas' day.
If it rains on the 8th of June, it foretells a wet harvest.
Saint Medard is to northern France what Saint Swithin is to England - a marker for either continuing wet or continuing fine weather. The biblical 'forty days' comes into play for both, and the only difference (besides the month and the legends) is that St. Swithin doesn't have a caveat to fall back on - no "St. Barnabas" to hold accountable if the tradition doesn't follow through.
Saint Medard was, according to his legend, a very pious boy, who gave away everything he could to the poor and needy, even if it wasn't his to give. On one of these occasions, a heavy rain fell, drenching everyone in the vicinity - except Medard. An eagle hovered over his head, sheltering him from the rain, a sign of Heaven's approbation. From this, farmers in northern France prayed to him for good weather and to protect their harvests, especially the haymaking, from rain.
As an adult, Medard was consecrated bishop of Vermand, but found it expedient to move his See about 25 miles southeast to Noyon. This was a time of concerted attacks by barbarians (as well as infighting among the Merovingian heirs of Clovis); Noyon had a strong wall and defenses, Vermand did not.
Medard seems to have been an exemplary bishop, continuing his good works while devoting himself to his heavy duties. He was a good friend and advisor to King Clotaire; when he died in 545, the king ordered a magnificent funeral for him and buried him at his own manor of Crouy near the Merovingian royal city of Soissons. Some twelve years later, Clotaire erected the Benedictine Abbey of Saint Medard over his tomb, where he, himself, was buried in 561.