Sainsbury's, Tesco and Marks & Spencer are set to report a decline in Christmas sales this week.The three retailers are widely expected to post disappointing festive figures, with Sainsbury's and Tesco bearing the brunt of a decline in food sales, while M&S suffered from a struggling clothing business.On Thursday, Tesco will release a trading update in which new chief executive Dave Lewis is expected to outline plans to cut hundreds of millions of pounds of costs and sell off assets, in a bid to implement price cuts in its stores and improve its balance sheet.Lewis could also formally announce Tesco's intention to sell off its online streaming business Blinkbox, a move which has been anticipated since he took over."I would be surprised if there is not a cut in capital expenditure. I don't see how you can justify spending £2.1bn a year," said Shore Capital analyst Clive Black, who added he expected Tesco to unveil a "focus on simplification" and a "material contraction of head office".Black said that the festive season highlighted a drastic change in the habits of British shoppers, who no longer seem to favour shopping at superstores."Five or six years ago people queued to get into superstore car parks. Those days are gone. For some people they are a glorified top-up shop," Black said.According to a Deutsche Bank estimate, Tesco is expected to post a 5% drop in like-for-like sales in the three months to 30 November, a slight improvement from a 5.4% decline in the last quarter, with a drop of 4.3% for the period since then.However, Black believes the retailer could fare better and has forecast a drop in like-for-like sales of between 2% and 2.5% for the Christmas period, slightly worse than Sainsbury's, which is expected to post a 1.8% drop in like-for-like sales.Meanwhile, M&S is expected to report a 0.9% increase in like-for-like food sales during the Christmas period but its like-for-like sales in the general merchandise division are expected to be down 3%.