More than 50,000 donations are needed between now and Labour Day to meet constant demand from hospitals and keep blood inventories at acceptable levels, the group said.
A spokeswoman for the group said the period from mid-July to the end of August is usually "the biggest challenge of the year" partly because many of their regular donors are travelling.
"We definitely need people to take action so that we can build up our national reserves at times like this," said Veronica Magee.
She said the organization — which manages the national supply of blood and blood products — aims to have about five to eight days worth of blood units on hand as a backup.
Right now, she said, they only have about four days worth of blood and are drawing on their national reserves.
"Every summer is a challenge," said Susan Matsumoto, the executive director of donor and clinic services, "but this year the trend is especially concerning."
The worst blood shortage the country has experienced in recent history was in the summer of 2008, when the group had to issue an urgent advisory.
As an effort to combat the shortage, the group has extended its Blood Signal rally until Sept. 3 as well as extending hours at its existing clinics and adding special clinics in selected locations.
Magee said it doesn't just take one person to help another — it can take 50 donors to help one car crash victim, up to five donors to help save someone undergoing cardiovascular surgery and up to five donors a week to help someone undergoing cancer treatment.
"It takes so many more people to help save the life of one person," Magee said.
About 50 per cent of the blood the organization collects nationally comes from Ontario.
Blood collected goes first to hospitals in the area where it was donated, with any extra being sent to other areas in need, Magee said.
Canadian Blood Services operates 42 permanent collection sites and more than 20,000 donor clinics across the country, with the exception of Quebec.