If you need to get blood drawn for lab work, don't go during off hours. I recently needed to get some blood drawn so I figured I'd go the hospital at 9am on a Sunday when the lab opened. My thinking was that there'd be nobody there so I wouldn't have to wait. I was right, I didn't have to wait. But it was absolutely the worst blood draw ever. The needle hurt the entire time it was in and I ended up with a huge bruise that lasted for days.

Looking back on it, I'm guessing the most experienced people, who are probably the best at drawing blood in a pain-free manner, are very unlikely to be in the lab at 9am on a Sunday, or any other off-hours for that matter. The people with the most experience are going get the first shot at their shifts, and they'll probably choose 9-5 on the weekdays. If you need to get blood drawn, that's when you should go. I can tell you from experience that getting your blood drawn by a rookie leaves a lasting impression.