In order to change the "ALL-DAY" text on iOS you should create a custom renderer. For more details regarding Calendar Custom Renderer on iOS please check our help article: Calendar iOS Custom Renderer.
Basically, you will need to create a class inside the iOS project that inherits from Telerik.XamarinForms.InputRenderer.iOS.CalendarRenderer, override the OnElementChanged method and inside it you could change the text of the AllDayEventsView LabelView. For more details please check the CustomCalendarRenderer.cs
file inside the iOS project from the attached file.
I hope I was helpful.
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