TOKYO — Traffic jams up to 28 miles long are forecast when Japan celebrates the Obon holidays, with some flights and high-speed trains already sold out.

Held Aug. 13-16, Obon is a Buddhist festival when families get together to welcome back and worship their ancestors’ spirits. For many Japanese, it stretches to include two weekends, starting Friday, with companies closing for a summer holiday.

Reservations on bullet trains are almost up 30 percent from last year, when many people refrained from traveling after a massive earthquake and tsunami devastated northern Japan and caused a nuclear disaster.

The Japan Road Traffic Information Center advised travelers to avoid highways leaving major cities this weekend and returning Aug. 14-15.

Okinawa, Hawaii and Guam are popular international destinations this year, according to Japan Airlines.