endpoints(data, on = 'months', k = 1) - 1

d = seq(as.Date("2004-02-20"),as.Date("2004-12-20"), by = 'day')
i = 6     #倒数第6天
aggregate(d,list(months(d)),function(x){x[length(x)-i+1]})$x   #随便什么分组函数都可以

找不到你的附件。简单说一下我的思路：
用dplyr包，按月group，按日倒序排列，用nth()取第6个数据。

library(dplyr)
library(nycflights13)

#Prepare an sample data set;

sample<-flights%>%
  select(year,month,day)%>%
  group_by(year,month,day)%>%
  filter(row_number() == 1)

#Keep only the 6th day from the bottom of each month
  
final<-sample%>%
  group_by(year,month)%>%
  arrange(-day)%>%
  filter(row_number()==6)

能给一个怎么取每月最后一个数的code吗