I lived in an area near a beach. Every year my neighbor would throw bread in the water as part of a ritual. I have a Jewish background but never saw this done before.
We no longer live there, but I still wonder why this was done.
Tashlikh is a lovely practice for Rosh Hashanah where we symbolically cast on the water all that we've got wrong over the previous year - marking our intention to let those things flow away (it has to be moving water) and allow a better year to come. It's not unlike the notion of secular "New Year's resolutions". My own community go each year, after our communal Rosh Hashanah lunch, to a nearby lake that a river flows through. The ducks certainly appreciate our efforts - and we find it meaningful and a great way to start the new year.
answer: this was likely done on Rosh Hashanah - it is the symbolic "tossing away" of sin from one's life in preparation for Yom Kippur.
"Another popular practice of the holiday is Tashlikh ("casting off"). We walk to flowing water, such as a creek or river, on the afternoon of the first day and empty our pockets into the river, symbolically casting off our sins. Small pieces of bread are commonly put in the pocket to cast off. This practice is not discussed in the Bible, but is a long-standing custom. Tashlikh is normally observed on the afternoon of the first day, before afternoon services. When the first day occurs on Shabbat, many synagogues observe Tashlikh on Sunday afternoon, to avoid carrying (the bread) on Shabbat."