Nikkei sees biggest 1-day gain in over 3 weeks

*Nikkei marks biggest one-day percentage gain in 3 weeks

*Window-dressing, reassurance on U.S. economy prompt buying

*Fujifilm plunges to biggest percentage loss in 21 years

*Nikkei up on week, but down for third consecutive month (Adds stocks, details)

By Elaine Lies

TOKYO, Aug 29 (Reuters) - Japan's Nikkei stock average climbed 2.4 percent on Friday, its biggest one-day percentage gain in over three weeks, as Canon Inc and other exporters rose after surprisingly strong U.S. GDP data temporarily eased fears about its economic health. Banks climbed as well, with top lender Mitsubishi UFJ Financial Group and its peers rising after U.S. banks gained on the brighter economic outlook and a management shake-up at top mortgage financial company Fannie Mae . But Fujifilm Holdings plunged 12.4 percent, its biggest one day loss in 21 years, after the company slashed its profit forecast by almost a quarter due to higher raw materials costs and a sharp price fall for digital cameras.

The Nikkei rose 3.2 percent on the week but slid 2.3 percent for the month, its third consecutive month in negative territory. Market players were sceptical about Friday's gains.

"Yes, the U.S. GDP figures were good, but this is old data, as of the end of June," said Masayoshi Okamoto, head of dealing at Jujiya Securities. "The Nikkei has responded strongly but we can't say it's gained on reasons that are sustainable. Everything could easily be erased next week."

Others pointed out that fund managers were buying shares to increase the value of their positions before closing books for the month.

"What we're really seeing here is month-end window-dressing," said Hideyuki Ishiguro in the investment strategy department of Okasan Securities.

The benchmark Nikkei .N225 gained 304.62 points to 13,072.87 and marked its greatest one-day percentage gain since August 6.

The broader Topix rose 2.9 percent to 1,254.71.

FUJIFILM FADES

Most market players were lukewarm about a government economic package to be unveiled later on Friday.

"It's not something that's going to send the Nikkei up 200 points, though some sectors could gain a bit depending on the contents," Jujiya's Okamoto said.

"If the market ends up focusing on the package, that's mainly just because there are no other real factors."

The day's biggest loser was Fujifilm, whose tumble was set off by Thursday's announcement that it expected its operating profit to be 160 billion yen ($1.5 billion) in the year ending March 2009, some 24 percent below its earlier forecast and a Reuters consensus estimate.

"With the economy in such bad shape, Fuji is likely to face tough times ahead, especially given its dependence on the European market, where currency rates have affected it as well," said Mitsushige Akino, chief fund manager at Ichiyoshi Investment Management.

But investors were equally quick to seize on good news, sending Nintendo Co surging 8.4 percent to 51,800 yen after it raised its full-year operating profit outlook by 23 percent and hiked its dividend forecast on Friday, citing strong sales of its Wii video game console and DS handheld games.

Exporters climbed, with Canon gaining 3.3 percent to 4,950 yen. Honda Motor Co rose 2.6 percent to 3,580 yen and Toyota Motor Co rose 3.4 percent to 4,930 yen.

Mitsubishi UFJ climbed 3.6 percent to 839 yen, while Mizuho Financial Group rose 4 percent to 470,000 yen and Sumitomo Mitsui Financial Group gained 4.2 percent to 670,000 yen.

Trade picked up, with 1.78 billion shares changing hands, compared with last week's daily average of 1.59 billion. (Reporting by Elaine Lies; editing by Sophie Hardach)