By Martinne Geller

NEW YORK (Reuters) - Top U.S. retailers will give investors the most detailed picture yet of holiday sales this week, with most expected to show better sales after an abysmal 2008 performance.

The retailers will report sales at stores open at least a year, data that may also shed light on their positions heading into the spring season. Wall Street is predicting a tougher environment as consumers return to saving money.

Chains from Costco Wholesale Corp to Macy's Inc to Abercrombie & Fitch will report December same-store sales data Wednesday and Thursday. Analysts expect a 1.3 percent increase from a year earlier at 30 retailers tracked by Thomson Reuters Data.

That would mark the fourth consecutive monthly sales increase, after a straight year's worth of declines during the recession.

Jharonne Martis, director of consumer research for Thomson Reuters, said the December results should reflect an improvement from last year, when same-store sales fell 3.6 percent, but not an economic recovery.

A rise of 1.3 percent "is still on the weak side, but it's an improvement from last year," Martis said. "Things are better, but they're still not healthy yet."

Retailers can ring in up to 40 percent of annual revenue in the weeks leading up to Christmas on December 25. This year, as investors search for signs of an economic recovery, the period holds even greater significance.

"Santa did show up this year, delivered a lot of presents, and helped retailers become at least as profitable, and likely more profitable for everyone, than last year," said Stifel Nicolaus analyst Richard Jaffe. "Things are going to be OK. "Not great, but OK."

Industry groups such as the International Council of Shopping Centers (ICSC), ShopperTrak and SpendingPulse have released preliminary data on holiday sales, but their parameters often differ, leading to widely varying figures.

ICSC has forecast an increase of about 2 percent for December. The National Retail Federation expects holiday sales for November-December to decline 1 percent.

This week's same-store sales results will be the first figures provided by retailers themselves.

"I absolutely trust the same-store sales numbers because those are what they are," said Patricia Edwards, founder of wealth management firm Storehouse Partners.

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At the start of the holiday season after U.S. Thanksgiving, shoppers focused only on big bargains. As a result, retailers clocked a weaker-than-expected 0.5 percent gain in November, dimming hopes for a recovery and pushing retail shares lower.