languagetool-org/languagetool · error

Could not find day of week for '<dayStr>'

Error message

Could not find day of week for '<dayStr>'

What it means

The Dutch DateCheckFilter maps Dutch weekday tokens (abbreviations ma/di/wo/do/vr/za and full names maandag…zaterdag) to Calendar constants. It throws this RuntimeException when the day string matches none of them. Notably there is no Sunday ('zo'/'zondag') branch, so any Sunday token reaching this method throws.

Source

Thrown at languagetool-language-modules/nl/src/main/java/org/languagetool/rules/nl/DateCheckFilter.java:49

  @Override
  protected Calendar getCalendar() {
    return Calendar.getInstance(Locale.UK);
  }


  @SuppressWarnings("ControlFlowStatementWithoutBraces")
  @Override
  protected int getDayOfWeek(String dayStr) {
    String day = dayStr.toLowerCase();
    if (day.equals("zo") || day.equals("zondag")) return Calendar.SUNDAY;
    if (day.equals("ma") || day.equals("maandag")) return Calendar.MONDAY;
    if (day.equals("di") || day.equals("dinsdag")) return Calendar.TUESDAY;
    if (day.equals("wo") || day.equals("woensdag")) return Calendar.WEDNESDAY;
    if (day.equals("do") || day.equals("donderdag")) return Calendar.THURSDAY;
    if (day.equals("vr") || day.equals("vrijdag")) return Calendar.FRIDAY;
    if (day.equals("za") || day.equals("zaterdag")) return Calendar.SATURDAY;
    throw new RuntimeException("Could not find day of week for '" + dayStr + "'");
  }

  @SuppressWarnings("ControlFlowStatementWithoutBraces")
  @Override
  protected String getDayOfWeek(Calendar date) {
    String englishDay = date.getDisplayName(Calendar.DAY_OF_WEEK, Calendar.LONG, Locale.UK);
    if (englishDay.equals("Sunday")) return "zondag";
    if (englishDay.equals("Monday")) return "maandag";
    if (englishDay.equals("Tuesday")) return "dinsdag";
    if (englishDay.equals("Wednesday")) return "woensdag";
    if (englishDay.equals("Thursday")) return "donderdag";
    if (englishDay.equals("Friday")) return "vrijdag";
    if (englishDay.equals("Saturday")) return "zaterdag";
    return "";
  }

  @SuppressWarnings({"ControlFlowStatementWithoutBraces", "MagicNumber"})
  @Override

View on GitHub (pinned to 2e990059ce)

Solutions

1. Add Sunday mapping: if (day.equals("zo") || day.equals("zondag")) return Calendar.SUNDAY;
2. Check the message's day string and make sure the caller passes the lowercased, trimmed token
3. Update the rule regex so it only emits tokens covered by the mapping

Example fix

// before
if (day.equals("za") || day.equals("zaterdag")) return Calendar.SATURDAY;
throw new RuntimeException("Could not find day of week for '" + dayStr + "'");
// after
if (day.equals("za") || day.equals("zaterdag")) return Calendar.SATURDAY;
if (day.equals("zo") || day.equals("zondag")) return Calendar.SUNDAY;
throw new RuntimeException("Could not find day of week for '" + dayStr + "'");

Set<String> known = Set.of("ma","di","wo","do","vr","za","maandag","dinsdag","woensdag","donderdag","vrijdag","zaterdag");
if (!known.contains(dayStr.toLowerCase())) throw new IllegalArgumentException("Unrecognized Dutch weekday: " + dayStr);

boolean isDutchWeekday(String s) { String d = s == null ? "" : s.toLowerCase(); return Set.of("ma","di","wo","do","vr","za","maandag","dinsdag","woensdag","donderdag","vrijdag","zaterdag").contains(d); }

try { day = filter.getDayOfWeek(dayStr); } catch (RuntimeException e) { log.debug("Skipping unmatched Dutch weekday {}", dayStr); }

When it happens

Trigger: A Dutch date rule matches 'zondag' or 'zo' and calls getDayOfWeek, which has no Sunday mapping; or the token differs from the exact equals() strings (case differences, extra characters).

Common situations: Dates in text containing Sunday matched by the rule; weekday abbreviations in different case since the filter uses exact equals, not startsWith.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Could not find day of week for '<dayStr>'
• Could not find month '<monthStr>'
• Could not find month '<monthStr>'
• Could not find day of week for '<dayStr>'
• Could not find month '<monthStr>'

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/5ec3b516327c1422. Report an issue: GitHub.