Won't your first line mean every Monday in May as well as days 25 to 31 of May?

At least busybox's cron implements it that way:

  if (line->cl_Mins[ptm->tm_min]
  && line->cl_Hrs[ptm->tm_hour]
  && (line->cl_Days[ptm->tm_mday] || line->cl_Dow[ptm->tm_wday])
  && line->cl_Mons[ptm->tm_mon]
  ) {