Everyone leaves gifts for everyone else. For example, if there are eight families in the exchange, then you give a gift to each family (cabin) so you would be giving gifts to seven cabins since you would be the eighth cabin. When you give a gift to that cabin, you can either give one gift for the cabin or give a gift for each person in that cabin. For example, a cabin has one mother and two kids. You can leave one gift for all three to share or you can leave a gift for the mother, and then another gift for each kid. For instance in a past exchange, I left a key chain for the mom and Disney silly bandz for the kids. So you gift to the seven cabins and in turn the seven cabins gift to you.
You don't leave gifts all three days, just one day. You can choose whatever day you want to deliver gifts. I personally deliver late the first night but most participants tend to deliver throughout the second day of the cruise. You can wait to deliver on the last day, just remember to deliver before dinner because most people take down their FE's after dinner and pack the luggage by ten for the cabin stewards to pick up.
All participants will be given a list of cabin numbers so you know who exactly to deliver gifts to. I don't recommend just walking around looking for hanging FE's. For one that is a lot of walking and secondly, some guests do hang up FE's that aren't part of the exchange. They might be part of another Disney forum exchange or they just might like hanging up an FE. I just know on each cruise I have been on I have seen several FE's on cabins that weren't part of the exchange. I don't leave gifts for random FE's, I only leave gifts from the list.
Hope that makes sense. I tend to get wordy

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