There are duty hour restrictions now for interns and residents. They are not supposed to do more than 80 hours a week or the training program's accreditation can be at risk. Many trainees lie about it to stay in compliance and many (especially surgical specialties) work over 100 hours. This was based on a New York case (Libby Zion) where an overworked resident was blamed for a math error that led to a large dosing error of medication on a pediatric patient. The fatigue was likely unrelated to the medical error, but it shined a light on the work hour issue for residents (That is why it is called a residency-because you were there more than your home). So, duty hours restrictions were enacted about 15 years ago. It is a double-edged sword, because now you get residents who graduate with 30-40% less experience based simply on the number of hours they worked. So they are entering practice with fewer hours of experience, having not seen as much pathology as their predecessors did. In addition, there are no duty hours restrictions on physicians in practice, so in some settings, a newly minted physician fresh out of residency is thrown into a practice where they do 26 or 48 hour shifts and they have never had that experience or developed the stamina to withstand it. It can be a real eye opener for a new physician in some cases. The longest shift I did was 72 hours straight in an ICU as a resident. I had to cover for someone who needed off, so I was on in house call for 3 days straight. I routinely covered a VA emergency room for 48 hours in a row on weekends to get extra moonlighting money. Although I was required to be in house for the entire 48 hours, it was typically only working for about half of those hours. These usually occurred following my 12 hours shift on Friday at my daytime job, so it was really 60 hours straight. I don't do those types of hours any longer, thankfully, but there are still some who do.
So, the answer to your question is, physicians typically prefer to get all of their work done and over a weekend or a week at a time so that they are not constantly on call. For instance, a surgeon may take a week of call and it could be a light week and they don't work many nights. However, they could also have a bad week and work 3 or 4 nights in a row doing big surgeries into the night in addition to their normal daytime clinic and OR schedules. Thankfully, these bad weekends or weeks are not that common in most practices. But if your group is small, there are not many people to share call with, so some get stuck if partners are on vacation etc. For example, if you have a group of three persons, most doctors would prefer to work an entire weekend versus a portion of every weekend. That way, you have two completely free weekends out of three. You may get killed on your weekend, but maybe you don't? It is a gamble.
Larger groups tend to split it up so that their call is not so burdensome. Many ER shifts are 12-24 hours in duration. I am not ER, but in my department, we do 12-16 hour shifts. Some of the surgeons do longer, but they typically get breaks in between the demand for OR cases, so they just have to stay available. We cover enough services such that our shifts are typically non stop activity, and we are required to be in-house whereas the others have the freedom to go home or leave the hospital if they are not busy.