I'll start with I play on PC and I don't use any mods. I started a new character for the event, a single female living in Willow Creek. I was able to do the event through the planting of the flowers ...
Ideas
I'm having this issue too.
First: Quests worked for awhile, and then the game stopped showing them to me.
Second: With a family I got the newsletter but after reading it, it disappeared and the quest was still incomplete. Every other familiy didn't received the newsletter.
Every other type of mail arrives without problems.
The things I already tried to do:
I tried creating another world and it didn't work
I tried to eliminate and replace the mailbox
I tried to create another new family