Templates can be used only with field access, property access, single-dimension array index error

You can't use DisplayFor because the expression can't leverage that extension method, you just use the raw value:

@objclasstime.ReturnPersianDay(int.Parse(item.ClassDay))

instead of:

@Html.DisplayFor(modelItem => objclasstime.ReturnPersianDay(int.Parse(item.ClassDay)))

If you want to use a method with DisplayFor, just store the value in a variable before using it.

You had

@Html.DisplayFor(modelItem => objclasstime.ReturnPersianDay(int.Parse(item.ClassDay)))

Instead, try this

var persianDay = bjclasstime.ReturnPersianDay(int.Parse(item.ClassDay));
@Html.DisplayFor(m => persianDay)

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