Go inside the Phoenix Fire Department 911 call center to learn about what’s going on when you call 911 in the Phoenix metro area. Hannah Gaber/

The need for blood donations never takes a vacation, even when you do.

United Blood Services is hoping for at least 500 blood donors this Sunday at Arizona’s largest blood drive.

The Fourth of July week is the most challenging time during the summer to meet patients’ blood needs, according to Sue Thew, spokeswoman for United Blood Services.

The reason for this is that people tend to extend their vacations to longer than a three-day weekend, thus dramatically decreasing blood donations.

Currently, United Blood Services has only a one-day supply of O-negative blood, the universal blood donor. They strive to keep a three-day supply of all blood types.

Thew said with 500 blood donations, every hospital in the state will have enough blood for one day.

United Blood Services scheduled this event to prepare for the shortfall of blood they know is going to happen.

According to Thew, it takes between 24 and 36 hours to test and process the donated blood before it can be released to a hospital. So, it is important that donations come in before the decline in donations begins next week.

Only about 6 percent of the population has type O negative blood, Thew said, but it can be received by 100 percent of patients.

“Increased traffic means an increased potential for accidents,” she said. “We want to be ready.”

Emergency and trauma situations are where O-negative blood is immediately needed.

And when high school is not in session, blood donations slow.

Arizona’s largest blood drive is being held this Sunday at the Tempe Center for the Arts from 7 a.m. to 1 p.m.

It is encouraged that people interested in donating make appointments either on or by calling 1-877-827-4376.

The first 500 donors will receive a voucher good for any Diamondbacks baseball game this season  All donors will receive a voucher for a free burger from Whataburger  One donor will be one of the 10 finalists entered to win a Volkswagen Passat R-Line.

A few tips to follow before donating:

1. Eat a meal before you give.

2. Stay well-hydrated.

3. Bring a photo ID.

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