It is quite often that in locative you have to change vowel a -> e (obiad - obiedzie, sąsiad - sąsiedzie, las - lesie, świat - świecie)
You mean locative, right? Not instrumental? "o obiadzie" is not instrumental, right?
You're right, thanks for correcting - edited my comment.
But it's 'obiEdzie' :)
The acoustics on this sentence are terrible, even after I saw the answer, I could not understand the word zapominaja