Unlike many gods, Leto doesn’t have any surviving festivals. I have been working out a short modern festival calendar..but it is still a work in progress. It includes the festival of the Hyperborean maiden in January, and a festival of frogs in the later part of May. Technically she is honored at the birth of the twins whether that be the Boeotian birth in February or the Ionian birth in May.
She doesn’t have a historically prescribed day, although honoring her before the sunset ushering forth the Noumenia would be appropriate. I have also adopted Sannion’s suggestion fir the Starry Bull tradition to honor her on the second day of the month on which we usually honor the Agathos daimon. I really like this and anyone following these posts can readily understand exactly why as I consider her nature not unlike Agatha Tyche as a chthonic mother of plentitude blessing our homes. Very appropriate, given ger nature, to give her worship on this day!
That said, like her children, I ascribe the twilight hour to Leto as a time of wolves and transitions between states. Therefore it is at this time I would consider best for asking her blessing.