[smcl]

I was surprised when a normally-Russian-speaking Ukrainian colleague of mine mixed up months when trying to communicate in Ukrainian with another normally-Ukrainian-speaking Ukrainian - talking about something occurring in October but using "Listopad" (which I figured from Czech was actually November). I'd guess that there's enough similarities that you could have a reasonable conversation with someone when you know Russian and the other speaks Ukrainian ... but occasionally you'll encounter words which are just completely unintelligible and not even close.

[kislotnik]

Listopad is a Ukrainian word for October

[smcl]

I just double checked and it seems everywhere I look lists Listopad as meaning November, with October apparently being “Жовтень”. However there are a bunch of other Slavic languages which DO have October as something like Listopad (https://en.m.wikipedia.org/wiki/Slavic_calendar) so perhaps I shouldn’t be so sure of myself in future :-)