You can achieve your requirement by changing the timeline item templates. RadTimeline exposes two properties that you can use in order to change the appearance of the timeline items: TimelineItemTemplate and TimelineInstantItemTemplate. You can take a look at the Customizing Items help article where this approach is further described. Basically, you can create a custom DataTemplate and bound it to one of those properties depends on which item template you want to change. For your convenience, I have created a sample project demonstrating the above mentioned approach.
As another options for displaying the text you can use a tooltip. RadTimeline supports tooltips for its items. You can specify a tooltip template and display the details of the item. The Tooltips help article in our documentation will give you more information about this feature.
Hope this information is helpful.
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