Uh-ho, methinks I have issues. 0_o I got Cliff to a purple heart, so..

I guess you have to start hanging around Cliff more. He'll probably invite you after that.

Fall Wine Harvest is the 17th Fall. You can keep giving Cliff gifts, but you will have to wait until after you've landed him the job at the winery to experience more heart events (though I have heard of a glitch that allows you to experience the blue heart event on the morning of the wine job, the dialogue presumes Cliff has already been offered a job at the winery). You can still 'climb' up the heart ladder, but you cannot marry a bachleor until you have experienced all of their heart events.

There are additional requirements that must be met. Most predominant are that the heart of your guy is actually a deep, dark red (there is a noticeable difference between red and deep red) and that you will need to have bought the big bed from the shopping network (this can only be done after upgraded and when you've purchased the other stuff).

The earliest you are able to marry is Winter, Year One (with the exemption of Kai), due to you only being able to purchase products from the shopping channel one a week. There are also arguments about benefits of a later marriage, though personally I don't think there's much difference. The main difference I noticed is that if you wait until further on (Year Four) you will receive congratulations in the mail from other (finally married) couples in Mineral Town.