Nafplio > Delphi > Meteora in the middle of December

I have only been to Meteora in summer, so not too much help, but since you haven't got any other replies, maybe I can get you pointed in the right direction. Meteora is way up in the mountains and there is a good chance of snow. Here is an article from my recommended Meteora tour company: https://www.visitmeteora.travel/the-winter-is-here/

Delphi is also way up in the mountains which are higher than Meteora. There are ski lodges in the area. Of course it is possible to do the drive provided the weather holds. You are going to have to be flexible and have a plan B It is off season so Delphi will close at 3 pm so plan to arrive in the morning to get the most out of the day. The route you are proposing Athens to Nafplio is on good roads so should not be a problem. Do stop in Corinth there are lots of great sites in the area. Consider a stop at the canal, perhaps take the opportunity to visit Ancient Corinth and the wonderful Fortress at Acrocorinth and Ancient Nemia is also a good site. You are going to love Nafplio. It is a special place. Plan at least 2 or three nights there particularly if you want to do some of the nearby ancient sites.

Nafplio to Delphi is fairly easily done take the bridge at Patras that bridge is an attraction in its own right. You will need to watch the weather for this leg. Delphi to Meteora and return to Athens can be done in two days three would be better. Its a 5 hour drive back to Athens. Do rent the car at the airport so you don't need to drive in Athens. I can recommend ACR Athens car rental. We have used them 3 times and they are reliable carry full insurance but with a deductible. They deliver the car to you at the airport and you drop it off there on the return.

Here is what you will see on this leg of the trip but in summer.

Delphi and Meteora https://www.flickr.com/photos/stanbr54/albums/72157698930270992

Nafplio and Peloponnese http://www.flickr.com/photos/stanbr54/sets/72157632094108982/
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I have not been to Meteora but the other places I visited in early December. Our weather was good: chilly with some rain and some nice sun. In Delphi it was nice. Of course that varies on a daily basis...

It is very off-season. This is good and bad. It is good as there will be much fewer tourists and you can have sites to yourself. But without tourists, many places close up shop. Some hotels/restaurants will be closed so you may have to look around more. Less so in Nafplio but definitely in Delphi and probably Meteora. Sites also close early and the sun sets early.

With two weeks, you can easily see those sites and take your time doing it. You might want to add in Olympia on your way between Nafplio and Delphi. It makes a good overnight stop and isn't too far out of the way. Galixidi is also a nicer town to stay in than Delphi, although it is very quiet in December. Not far outside of Delphi is Hosias Loukas, a Byzantine monastery that is a beautiful peaceful place. A daytrip to Mystras from Nafplio is also worthwhile if you have several days in town.

If the weather will be ok, is it making any sense to make it this way? :

Athens to Meteora (2 nights)

Meteora to Arachova (2 night)

Arachova to Kalamata (3 nights)

Kalamata to Gytheio (2 nights)

Gytheio to Nafplio (2 nights)

Nafplio to Athens to my flight home.

*I know that Arachova is far away from Kalamata but there are few things that I want to see in this area and I don't want to stop in Nafplio after Arachova because I want to save it to the end of the trip so it will be easy to go Athens for my flight back.

BTW how can I know if there is fog on the roads according to the weather?

I do not understand the interest in Kalamata and Gythio in the winter .... llikely to be very deserted & only the locals & local spots open. Why? Meteora has landscapes.... Nafplio has a fascinating Old City, and because of Athenian visitors, it will have a good selection of cafes, tavernas, shopping & nightlife. But you need to explain why Kalamata/Gythio -- are you looking at photo's from a friend's summer visit? Do you have relatives there? Otherwise cannot fathom these choices.

Actually I read that it is recommended visiting these places year-round and use them as a base to visit:
Ancient Messene, Mystras, Sparti, Caves of Diros, Monemvasia , etc.. Isn't it right?
Also, I have 2 weeks, where can I spend them beside Meteora, Delphi, and Nafplion?

Any thought? Advice?

What else to do with part of your 2 weeks? Frankly, Athens is at its most vibrant in winter, yet I don't see your itinerary including a few days there. Have you already spent a long time there and have covered all its highlights?? Ive been to greece 13 x (yes Iknow but I don't go anywhere else), and each time am in Athens at start & end of my stay... an every single time, I discover new delights. And in winter, nothing will be crowded ... you can be all on your own in many museums and ancient sites. There will be concerts, art shows, great night-life, the list goes on. If any of this interests you, I could share a few of my favorite things....

I also went to Meteora this October, the drive from Meteora to Athens is 4 hours in Highway, super easy, and I am using google map with data,

I do see pictures of snow in Meteora, and after November the opening time is shorter.

But most of the islands is dead in Winter time, so your choice mainly is inland.

Yes sure the mainland, that's why I asked about this route:
Athens to Meteora (2 nights)

Meteora to Arachova (2 night)

Arachova to Kalamata (3 nights)

Kalamata to Gytheio (2 nights)

Gytheio to Nafplio (2 nights)

Nafplio to Athens to my flight home.

2) No problems at all, followed roughly the same route 2 weeks ago

3) I would recommend a stopover at the picturesque villages around Arachova, eg Agoriani and Antikira. The food is fantastic btw, try formaela cheese!