Please take two screenshots from your trade offer screen. Take them in browser rather than the Steam client, and do not crop or edit them in any way (the whole screen should be visible).
The first should show the trade offer, with your cursor hovering over the image of the offered item, demonstrating that it is the festivized variant. The second should show the trade offer, with your cursor hovering over the profile picture of the user offering it to you, so that their profile URL is displayed in the lower lefthand corner of the screen.
Upload both screens to imgur and post the links here.