# Calendar Systems – Day of Week & Date Info

> Calendar Systems – Day of Week & Date Info is a paid API for AI agents from calendar-systems.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-30).

Returns the weekday, day of year, quarter, ISO week, and ISO week-date for any date from year 1 to 9999, supporting both Gregorian and Julian calendars.

## Facts

- Endpoint: GET https://calendar-systems.x402tookit.com/day-of-week?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-30
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/calendar-systems-day-of-week-date-info-94f18116
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap__8Sp7ONz50ylErTBOl3QS

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability calendar-systems-day-of-week-date-info-94f18116
```

Example prompt: What day of the week is July 4, 2026, and what ISO week number and quarter does it fall in? Use the Gregorian calendar.

## When to prefer this

Choose this endpoint when you need comprehensive calendar metadata for a specific date — especially weekday, ISO week, quarter, and day-of-year — across both Gregorian and Julian calendar systems, covering the full historical range from year 1 to 9999. Prefer it over generic date libraries when you need ISO week-date conversion or Julian calendar support without writing custom code.

## Known failure modes

- Date outside year 1–9999 range returns an error
- Malformed date string (not YYYY-MM-DD) returns a validation error
- Invalid ISO week date format returns an error
- Invalid calendar enum value returns a validation error
- Missing required date or isoWeek parameter returns an error

## How this service works

Weekday of any date from year 1 to 9999 (Gregorian or Julian calendar) with day of year, quarter, ISO week and ISO week-date.

## Output

Returns a structured object containing: the weekday name, ISO weekday number, day of year, days left in year, quarter, ISO week number, ISO week year, ISO week-date string, month name, leap year flag, days in year, and optionally the equivalent Gregorian or Julian calendar date. Also includes a summary and disclaimer.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [],
     "properties": {
      "date": {
       "type": "string",
       "description": "YYYY-MM-DD, year 1 to 9999"
      },
      "isoWeek": {
       "type": "string",
       "description": "ISO week date like 2026-W01-4 to convert to a calendar date instead"
      },
      "calendar": {
       "enum": [
        "gregorian",
        "julian"
       ],
       "type": "string",
       "description": "gregorian (default, proleptic) or julian"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "date": {
       "type": [
        "null",
        "string"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "isoWeek": {
       "type": [
        "null",
        "number"
       ]
      },
      "quarter": {
       "type": [
        "null",
        "number"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "weekday": {
       "type": [
        "null",
        "string"
       ]
      },
      "calendar": {
       "type": [
        "null",
        "string"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "leapYear": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "dayOfYear": {
       "type": [
        "null",
        "number"
       ]
      },
      "monthName": {
       "type": [
        "null",
        "string"
       ]
      },
      "daysInYear": {
       "type": [
        "null",
        "number"
       ]
      },
 
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/calendar-systems-day-of-week-date-info-94f18116/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from calendar-systems.x402tookit.com](https://www.zero.xyz/host/calendar-systems.x402tookit.com/llms.txt)