填空题

Severe winter storms have worsened the nation’s usual post-holiday blood shortage, prompting an urgent call today for people to donate blood. Type O blood is the most urgently needed, because it is the universal blood type, meaning it can be safely transfused to most people in an emergency. "The severe winter weather has forced the cancellation of blood drives and limited our ability to collect blood," American Red Cross President Elizabeth Dole said. The Red Cross reported having less than one day’s supply of Type O blood on hand in 18 of the organization’s 38 collection regions. The Red Cross provides about half of the nation’s blood supply. America’s Blood Centers, which supplies most of the rest, also has reported shortages.
Reacting to the shortage, a Pittsburgh blood bank asked 40 hospitals in western Pennsylvania, West Virginia and Ohio to postpone elective surgeries through Wednesday. At least 1,000 donors are needed each day for the next several days to stabilize the supply, said Mark Lynch, spokesman for the Central Blood Bank. "We’re talking about (postponing) hip replacement, for example, and orthopedic cases. These are sick people who need surgery, but it’s not life threatening," he said. When its blood supply is stable, the blood bank needs about 700 donors each day to maintain normal inventory. The Pittsburgh blood bank last asked hospitals to postpone elective surgeries because of a blood shortage in October 1995. Such procedures were canceled for almost a week.
Nationwide, donations typically fall off during the holidays, when donors become too busy to give blood regularly. Bad weather increases shortages both by blocking blood drives and by causing increased blood demand to treat accident victims.

We can infer from the passage that blood shortage usually occur()

【参考答案】

after holidays