"At that time I will discuss what I feel are the important issues facing New York and the country. I will explain why I'm running," said Blakeman, who declined further comment.
A former Port Authority of New York commissioner, Blakeman had wanted to run for New York City mayor until Republican Mayor Michael Bloomberg decided to seek a third term.
Blakeman recently had visited political power brokers to drum up support for a race against the untested upstate Democrat. Gillibrand was appointed by Democratic Gov. David A. Paterson to fill the vacancy created when Hillary Rodham Clinton resigned to become U.S. Secretary of State.
With the faltering economy feeding anti-incumbent anger, Republicans see new hope of ousting Democratic office holders.
"I believe in 2010, we're going to have the wind to our back," said Nassau Republican chairman Joseph Mondello, whose party took back control of the Nassau Legislature, the county executive's office and the comptroller this fall.
"A person like Bruce Blakeman, who is good-looking and outspoken, can really make a mark," said Mondello, who will be at the 11:30 a.m. Sunday rally at the American Legion Hall in Valley Stream. "He's experienced. He knows the business. He's got the credentials and qualifications."
Bethany Lesser, a spokeswoman for Gillibrand, said, "The Senator looks forward to running a vigorous campaign on her record against any opponent."