If you enjoy the smooth taste of a fine whiskey, derive pleasure from touring a whiskey distillery, and have a spare day on your itinerary, then Suntory Yamazaki Whiskey Distillery is for you. The complex consists of a museum (covering the history of Suntory Whiskey), a souvenir shop, a bar ( with a huge selection of whiskeys) plus, and highlight of your visit, a free tour of the production process and an opportunity to sample some of their products.
But don't just come all the way to Yamazaki Town for the Suntory experience. There is a whole lot of other experiences and activities for you to take in. For example, if you appreciate art, then Asahi Beer Oyamazaki Villa Museum of Art is for you. But don't just take-in the fine art on display, spend time strolling around the well maintained grounds. What about culture, then Myokian Tea-House might be suitable for you (the Tea Ceremony here is very famous and one has to book well in advance). I won't forget to tell you about the Temples and Shrines that are a must, beginning with Hoshakuji Temple and Rikyu Hachiman Temple. There is a huge history attached to Yamazaki and the area is well signposted with information about the past and how & where you can experience it. After all this touring around you are feeling hungry. There is a great selection of restaurants and cafes (don't ask me to recommend one, they all have a great menu) to satisfy your tastes.
Here is a plan I will recommend for you (based on how I have done it, the several times I have been here). Arrive here early (Yamazaki is easilly accessible from anywhere in the Kansai Region.There is the J.R. Osaka to Kyoto Line and the Hankyu Osaka to Kyoto Line) (10am is a good time), via the JR Yamazaki Station (you can arrive via the Hankyu Oyamazaki Station, just 5-minutes walk away), turn right and take the narrow lane that runs parallel to the rail-lines. About 10-minutes along you will see some very-large brown buildings over the rail-crossing. Go to the gatehouse and book your tour for 2pm (that gives you adequite time for a tour of the surroundings) and return to the station. From here turn left and make your way to the next rail-crossing, cross and head-up the road in front of you. Your first destination will be the Hoshaku-ji Temple. From here take the track behind the complex to the Asahi Museum (the museum runs a free buss to/from both stations). From here return to the station area and visit the Rikyu Hachiman-gu Shrine (the shrine has an amazing history). It's now time to satisfy your hunger (important if you plan on doing the distillery tour) at any of the many eateries between the two stations (the Cafe Tabi-Tabi opposite the J.R.Yamazaki is my favorite).
Then it's off to the distillery and the highlight of your day. Take the museum tour and, if you have time, take a nip before your tour. There is English speaking guides there and, when it is time for your tour, you shall be called. All I can say is; enjoy,enjoy,enjoy.
In response to the big question -"when is a good time of year to visit Yamazaki"? - my answer would be - "any time of year is a good time, but some times of the year are better than others". Spring (April/May) and Autumn (October/November) are more recommended as most facilities would be open and the colors at these times make for some really-great photo opportunities.