In Polish, it is called "Wielki Piątek" which translates as "Great Friday". Simiarly; the week is "Wielki Tydzień", translates as "Great Week". I always understood it in the similar vein as the Jewish "High Holidays" as the most important observation of the year.

The discrepancy between "Holy Week" and "Good Friday" always irked me when I learned these terms in English.