How to create subset using mosaic for seasons

Question

So I am a total beginner. For my project, I am just using RStudio with mosaic.

I have a dataset with 10 variables, describing in which month of 2019 what kind of accident happend, who was involved (car, bike, ...) and its outcome (deadly, slight injury, heavy injury)

I want to combine all wintermonths (Dezember, Januar, Februar) in one subset (season winter) to be able to compare them with the non winter months

I used

data_winter <- select (Berlin, UMONAT=="Dezember" & UMONAT=="Januar" & UMONAT=="Februar")

But it just gives out:

Error: object 'UMONAT' not found

What am I doing wrong? Is there a better way to do it?

    "No", "No", "No", "No", "No", "No", "No", "No", "Yes", "No", 
    "No", "No", "No", "No", "No", "No", "No", "No", "No", "No", 
    "No", "No", "No", "Yes", "No", "No")), class = c("spec_tbl_df", 
"tbl_df", "tbl", "data.frame"), row.names = c(NA, -13389L), spec = structure(list(
    cols = list(UANZAHL = structure(list(), class = c("collector_double", 
    "collector")), UMONAT = structure(list(), class = c("collector_character", 
    "collector")), UWOCHENTAG = structure(list(), class = c("collector_character", 
    "collector")), UKATEGORIE = structure(list(), class = c("collector_character", 
    "collector")), IstRad = structure(list(), class = c("collector_character", 
    "collector")), IstPKW = structure(list(), class = c("collector_character", 
    "collector")), IstFuss = structure(list(), class = c("collector_character", 
    "collector")), IstKrad = structure(list(), class = c("collector_character", 
    "collector")), IstGkfz = structure(list(), class = c("collector_character", 
    "collector")), IstSonstige = structure(list(), class = c("collector_character", 
    "collector"))), default = structure(list(), class = c("collector_guess", 
    "collector")), skip = 1L), class = "col_spec"))