In Italian, it's called Sentiero Italia. The national trail of Italy literally crosses the whole country, starting from Trieste, through the entire Alpine Arc and Apennine mountain chain, to Sicily and Sardinia. The trail ends at Santa Teresa Gallura in Sardinia. The entire length of the route is 6,166 kilometres. If you fancy seeing the trail and don't feel like walking thousands of kilometres, you should know that the Grand Italian Trail is divided into 368 sections or legs, and you are free to start wherever you want and walk as much as you prefer.
The trail was initiated back in 1983 by a group of ambitious hikers. It was established in 1995 through the so-called "Walk Italy" event supported by the Italian Alpine Club. That year became memorable owing to the first thru-hikers of the Great Italian Trail. They completed it in eight months, even though the calculated time for a thru-hike trip is considered to be 350 days. The period of eight months is just perfect considering the hiking season in Italy that begins in March and continues into November.
Northern parts of the trail might be covered in snow from October to May, so this part would be best during the summer months. Southern territories are hot in summer, so spring and autumn are the best times to hike there.