How to convert date to the closest weekend (Saturday)

Base R option. First create a list of all days, then match it with weekdays and subtract it from 6 (as we want Saturday) to get how many days we need to add in the original date column.

all_days <- c("Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday")

#As @nicola mentioned this is locale dependent
#If your locale is not English you need weekdays in your current locale
#which you can manually write as shown above or do any one of the following

#all_days <- weekdays(seq(as.Date("2019-01-14"),by="day",length.out=7))
#OR
#all_days <- format(seq(as.Date("2019-01-14"),by="day",length.out=7), "%A")

df$EOW <- df$date + 6 - match(weekdays(df$date), all_days)

df
#        date week        EOW
#1 2014-08-20   34 2014-08-23
#2 2014-08-25   34 2014-08-30
#3 2014-10-08   41 2014-10-11

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Or lubridate has a function ceiling_date which when used with unit = "week" would return you the next "Sunday" so we subtract 1 day from it to get "Saturday" instead.

library(lubridate)
df$EOW <- ceiling_date(df$date, unit = "week") - 1