How to find weight during a specific time

Click on My Home, then under your picture click on the little scale / +. At the bottom of that page you'll see "edit previous entries", if you click on that you can then page through all your previous weights... depending on how often you weigh yourself it might take a while but you can go back to the beginning of your entered data (in my case I entered the previous year's data which predates my joining MFP, I was using a different app) Reports on the website go back a year.

(edited to add: this is all based on being on the website, not the app so that may change things!)