I'm playing nintinto DS Animal crossings and one of the cheats I found?

The cheat told me to get 50 fossils and identify them and sell them all to tom nook and he would give me a whistle I got 50 and sold them to nook and didn't get the whistle I'm wondering if I have to sell them to him on the same day or if it matters? I want to go to the other island there is my friend has done it before but she didn't remember the cheat so I found it. and it doesn't tell me if It has to be on the same day or through out the days