I recently helped a customer who was using the XamDateTimeRangeSlider, whereby he wanted the tickmarks to be spaced at the start of each week of a month (e.g., "For the month of January, I want the tick marks to be placed as start of every week. So the first four tick intervals will have the same length, and the last interval will only be 3 days."):

The customer noted that by using the following settings:

      timeSlider.TickMarks[0].NumberOfTickMarks = 3;
      and setting UseFrequency="False"...

"...The tickmarks are are drawn in regular intervals based on the number of tickmarks."

In this situation, the best approach is to add the TickMarkValues manually in code, as follows:

XAML markup:
<ig:XamDateTimeRangeSlider Name="xamDateTimeRangeSlider1"
                           MinValue="1/1/2011" 
                           MaxValue="1/31/2011"/>

with the Loaded event handled below:

SliderTickMarks<DateTime> stm = new SliderTickMarks<DateTime>();
stm.TickMarksValues.Add(new DateTime(2011, 1, 2));
stm.TickMarksValues.Add(new DateTime(2011, 1, 9));
stm.TickMarksValues.Add(new DateTime(2011, 1, 16));
stm.TickMarksValues.Add(new DateTime(2011, 1, 23));
stm.TickMarksValues.Add(new DateTime(2011, 1, 30));
xamDateTimeRangeSlider1.TickMarks.Add(stm);