Canadian Blood Services is putting out a call for donors as the summer season winds down.

On Friday it said it tries to maintain an eight to 10-day supply of all blood types—but right now it only has a two-day supply of O negative.

That’s why about 1,500 appointments are needed between now and the September long weekend.

While the drop in donors this time of year is not unusual, the organization said it’s always concerning.

“Every summer we typically see blood donations go down. Understandably, people are outside enjoying the summer weather, trying to get the last trip to the lake in, yet the need for blood remains constant,” said Mike Choi, Canadian Blood Services territory manager.

Earlier this week it said all negative blood types were approaching critically low levels.

You can book a donation appointment online.