Fast food fans may be smiling today at the prospect of getting a free McDonald's Egg McMuffin on National Breakfast Day, but execs at the chain likely have heartburn after very disappointing U.S. sales in February. McDonald's, which on Monday is giving away free Egg McMuffins globally to celebrate the company-created holiday, saw a 4% sales decline in the U.S. in February. Analysts were expecting a 0.7% drop after two months of gains that stoked optimism that a simpler menu and the burger chain's new 'Lovin It' marketing campaign were helping the company reconnect with disenchanted customers. However, one bright spot for McDonald's continued to be breakfast, which accounts for about 25% of McDonald's U.S. sales. 'Breakfast continues to be our strongest daypart, and [sales] were flat in February', said McDonald's spokeswoman Becca Hary via email to TheStreet, in part because of the severe winter weather. TheStreet’s Brian Sozzi reports.