Oh, sorry, I somehow missed that part.
She asks you normally to go to the park wearing her usual outfit in the kitchen, once the scene for the park starts, she's wearing the sundress showing that CG you uploaded, then asks you if you could take a walk with her, and she has some dialogue, how she feels about you and that the MC makes her happy, at which point you can kiss her, there is an option to say that "you were hoping she would feel this way", when you do select that option, she starts a line, though doesn't finish it, something like:
"Oh really, I thought since you...nevermind" something along those lines (maybe hinting at some kind of consequence of having slept with many other girls, if that is the case, would be nice to have alternate dialogue there if the player hasn't slept with anyone else.), then she carries on saying she likes to spend time with you and that she would like to do this again sometime. Beyond that, I'm not sure what is the conditions for triggering the other two CGs of her wearing the Sundress though, waiting for a kiss and lifting up her dress for you.
Regarding the anniversary scene, do you only get that scene if Josef isn't around on the anniversary day? If so, that would explain why I haven't seen that scene yet, as Josef is still around in my playthrough(s), since I'm not content with the options available to get rid of him. (They all feel a bit unjustified/underwhelming, aside from the alleged treatment Anna got in their marriage from him, none of them feel well thought out, feels like a lot is left unsaid in the options available to really justify the actions of the MC to make him leave imo.)