The Easter Bunny showed up a week earlier this year and gave merchants a whopping increase in March business.

Double-digit sales gains were reported by several big retail chains, raising hopes that a rebound in consumer spending will help stimulate economic recovery. Retailers admitted, however, that part of the sales increases resulted because Easter Sunday fell on April 3 instead of April 11 as in 1981. Thus Easter shopping was done mostly in March rather than April.

Jeffrey Edelman, an analyst at Dean Witter Reynolds Inc., said the real test of whether a consumer-led recovery is under way will be to compare combined March and April sales of this year and last.

Sears, the nation's largest retailer, said sales of its merchandise group in the month ended April 2 rose 5.8 percent to $1.85 billion from $1.75 billion a year ago, taking account of 21 stores closed since then.

Sales in the first nine weeks of its fiscal year were up 5.2 percent to $3.13 billion from $2.97 billion.

K mart Corp., the second-largest retailer, said sales in the five weeks ended March 30 rose 13.3 percent to $1.54 billion from $1.36 billion a year earlier, accounting for eight store closings in the last year. In its first nine weeks, it posted an 11.4 percent sales gain to $2.54 billion from $2.28 billion.

J. C. Penney, No. 3 in size, said sales at its stores and catalogue operations rose 6.7 percent in the five weeks ended April 2 to $873 million from $818 million. For the two months of its fiscal year, Penney's said volume advanced 3.4 percent to $1.44 billion from $1.39 billion.

No. 4 F. W. Woolworth Co. said sales last month were up 9.2 percent to $441 million from $404 million in March 1982. For the first nine weeks, sales rose 6 percent to $745 million from $703 million a year earlier.

No. 5 Federated Department Stores said sales in the five weeks ended April 2 were $728 million, a 17.1 percent gain over $621 a year ago. Federated's sales for the nine weeks were $1.24 billion, up 15.5 percent from $1.08 billion.

Montgomery Ward, No. 6, said five-week sales were $467 million, up 2.5 percent from $456 million. Sales in the nine weeks were $788 million, up 2.2 percent from $771 million.

Seventh-ranked Dayton Hudson Corp. showed the strongest performance of the major retailers, a 26.6 percent sales gain to $560 million in the month, up from $442 million. Nine-week sales were up 25 percent to $906 million from $725 million.

No. 8 May Department Stores, parent of the Hecht Co. here, said March sales rose 19 percent to $346 million from $291 million. For the two months, sales rose 17.4 percent to $569 million.