The picnic is an artifact moment, mc tells about his mother death, he does that with every li, but with Jill it is so important!!!
That's weird to me too. The MC, if Sage's fuckbuddy and/or is on Jill's path, can tell either of them at the respective hang out what his feelings are on his birthday. In contrast, Josy, Maya, and Isabella do not have the opportunity to hear that from the MC at any time. This implies to me so that either Fuckbuddy Sage or Jill, are preferred to all other LI's.
However, you can balance all the LI's in that no LI gets to hear about these very personal feelings about their birthday.
In the official Walkthrought there is also something strange regarding the birthday. In EP5, after the MC has spent the night at Isabella's, the MC can call his father in the Freeroam event, who informs the MC about his birthday. So far too good. But in the Walkthrought it is not pointed out for the MC to text with his father and this is strange in that DPC writes in his Walkthrought exactly who the MC should communicate with via cell phone and/or Rooster. The joke is, even if the MC doesn't call Neil and thus forgets his own birthday, Arieth wishes the MC Happy Birthday.
The MC's birthday and knowing about his personal feelings on that day, seem to have a wider meaning for BaDIK. Now when I think of the story of Derek while shopping in EP8, where Becky sends a birthday greeting as an icebreaker to one of the two brothers, Rick or Paulie, I wonder if Arieth's Happy birthday was the same and if it was, how does Arieth know when the MC's birthday is?