Major retailers and shipping companies are gearing up for customers’ last-minute online orders to make sure they receive packages by Christmas Day.

To guarantee delivery of cards, letters and packages by the holiday, the United States Postal Service is urging customers to ship their items by Saturday for first-class mail, by Monday for priority mail and Wednesday for express mail.

FedEx said the last day to ship with FedEx Express for pre-Christmas delivery is Wednesday.

UPS said Friday is its shipping deadline for three-day delivery, Tuesday for two-day delivery and Wednesday for next-day delivery.

Close to 1,000 retailers, including Walmart, Target, JCPenney and Express, are offering free shipping with guaranteed delivery by Christmas Eve for online orders placed by Friday, also known as Free Shipping Day.

“We are entering the procrastinators’ window with people buying up against the deadline,” said Danielle Conte, a Centerport-based retail consultant and founder of customer shopping experience blog YoutailRetail.com.

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FedEx forecasts its shipments between Black Friday and Christmas Eve will total more than 317 million packages, a 12.4 percent increase over last year. UPS expects shipments to total more than 630 million packages between Black Friday and New Year’s, an increase of more than 10 percent.

This holiday shipping period includes an extra day compared to last Christmas because the holiday falls on a Friday, said Steve Osburn, a supply chain expert for global management consulting firm Kurt Salmon in Manhattan. Still customers should strive to make their orders a day or two before the retailers’ guaranteed ship date, he said.

“It is a network that it is already at capacity,” said Osburn, adding retailers won’t offer as many deals as last year to prevent delivery problems. “The unseasonal warm weather ensures carriers will make good on deliveries, but any last-minute surge or last-minute sale could throw a wrench into things.”

In order to get free delivery before Christmas, Toys R Us customers must place their orders by Saturday, Walmart and JCPenney customers by Sunday, Target customers by Monday, and Kmart and Best Buy by Tuesday. Amazon is offering standard shipping for orders made by Friday or free two-day shipping with Amazon Prime. All the retailers offer expedited options after those deadlines.

To prepare for the rush, 50 percent of retailers are offering customers the option to buy online and pick up in store, compared to 38 percent last year, Osburn said.