Sorry for being such a nit picker but you have made few mistakes in your post.
Grudzien...yes it means December..the root words for it are derived from Grud, a Russian word
Why would we have to borrow this name from Russian, if the Rusians themselves use the word "декабрь" for December, transliterated dekábr', derived from number 10? There is a word "gruda" in Polish, which means a frozen ground, or a clod (clod translates back to "gruda", "grudka", "gruda ziemi").
Etymology: "grudzień" or "grudnia" - used as a name for the 12th month since 15th c. in Poland. It comes from the Old Slavonic "grudьńь" - meaning a period when the ground is frozen, covered with clods. Other old Polish names of the 12th month of the year are:
+ prosień - from an old Slavic god of millet (proso in Polish), a protector of livestock
+ prosieniec - from the verb "sijać" (to shine, to radiate). The sun rays reflect from the snow covered ground. All appears lighter, brighter.
+ listopad - now November, a period when leaves fall
+ gódnik - used also in modern Kashubian, from the word Gody - old name for Christmas
+ jadwent and jadwient - folk version of the word adwent, advent in English
given the non-Gregorian calendar name of Grudzien.
Non-Gregorian? It is actually the last, 12th month the year, according to Gregorian calendar. You must have had in mind the Old Roman calendar (or calendar of Romulus). It had only 10 months, with the last month December (from dece, ten), and 304 days. The missing days between December and Martius (the first month) were unassigned.
Most European languages 'incorrectly' borrowed the December name from Roman calendar to Julian and then to Gregorian calendars.
Only few European languages keep their own name of this month, having nothing to do with number 10:
Belarusian: сне́жань, transliterated as snežanʹ (a month of snowing)
Ukrainian: грудень, transliterated as gruden' (same as Polish grudzień)
Polish: grudzień
Lithuanian: gruodis
Croatian: prosinac
Czech: prosinec - meanings:
1. of old Czech prosiněti - flashes of sun between clouds;
2. of old Czech siný - livid (ie blue-gray);
3. from the word pig (Polish prosię) (slaughter time of pigs)
Finnish: joulukuu