I have an example here of my spreadsheet.
Basically my main page has a drop-down (via data validation) where you can toggle through any month and it responds by showing data for that month.
What I want to do is be able to type a note for a specific day and have it show up in Sheet1!B2:B where the date in Sheet2!A2:A matches Sheet1!A2:A. So Sheet2 will have any number of notes across any month of the year. If there is a duplicate date in Sheet2!A:A then it could just grab the first or throw error (this will be an edge case). I would even be fine populating Sheet2!A2:A with all 365 days if it would make things easier.
What would be best is if I could just type my change in Sheet1!B2:B and it would grab the date from Sheet1!A2:A and make the entry in Sheet2, and then sync changes both ways. I know nothing about Google Docs scripts though.