There are many different relative time ranges supported but I don’t think there’s a way to do this specific thing, other than to use the datepicker and simply select the date as the previous Friday in a range.
There are a lot of cultural differences here between locations in the world, and it’s not at all obvious that if you’re on a Monday, that the previous Friday was the last work day. In the case of the “fiscal year” relative range, there’s even an option in Grafana to change the start of the fiscal year, because commonly people have different ones. Setting to “the last work day” is a very locally contextual setting that I imagine would be pretty hard for Grafana to support in a general way.
This related thread though shows some advanced tricks with the time range specifier, and you might find some help there, though the solutions I see there are not exactly what you’re after