I always thought (and so far experienced) that all heart-eventstake place automaticall once you met all the qualifiactions andentered the right place at the right timeat theright day.
But I have been trying to trigger the yellow heart event for Cliff several times now - he's been red for ages - but to no succeed. No matter how many times I enter the winery between 5 and 7PM, the event does not start.
Is there maybe a rival-event I should see first , or does it have to be a rainy day or anything ?

Walk into the Winery's house on Sunday, Tuesday, or Thursday between 5:00 pm and 7:00 pm. Inside you'll find Duke and Manna gossiping to each other and then Cliff and you would have a dinner.
Hope this helps!

Nope,thank you but that was the information I already had. By now I may add that it worked in the meantime, ON A RAINY DAY !!

Oh it did? On a rainy day?
I guess you must always try and try. It's really hard to make a red heart without all the events and then you try and wait for them. That happened to me to so I just lowered his heart color and married Kai instead.