# Autumnal Equinox Day API

> For the complete documentation index, see [llms.txt](/llms.txt).

Return autumn equinox date, UTC time, seasonal markers, and optional countdown data.

Use this endpoint to add the tool to your app, site, workflow, or agent without rebuilding formulas, validation, examples, and docs.

- Slug: `autumnal-equinox-day`
- Category: `datetime`
- Version: `2026-04-22`
- Credits per successful call: 1
- Starts on: free
- Max payload: 65536 bytes
- Privacy mode: `hash_only`
- Available on: free, starter, pro, business, scale
- Browser tool: https://miniwebtool.com/autumnal-equinox-day/

## Endpoint

`POST https://api.miniwebtool.com/v1/tools/autumnal-equinox-day/run`

## Request body

| Field | Type | Required | Description |
| --- | --- | --- | --- |
| `year` | `int` | no |  |
| `hemisphere` | `str` | no | north | south Choices: north, south |
| `as_of_utc` | `str | None` | no | Optional ISO datetime for countdown |

## Example request

```json
{}
```

## cURL

```bash
curl -X POST https://api.miniwebtool.com/v1/tools/autumnal-equinox-day/run \
  -H 'Authorization: Bearer mwt_live_YOUR_KEY' \
  -H 'Content-Type: application/json' \
  -d '{}'
```

## Response envelope

```json
{
  "request_id": "01K...",
  "tool": "autumnal-equinox-day",
  "tool_version": "2026-04-22",
  "credits_used": 1,
  "result": {}
}
```

## Errors

Errors are RFC 7807 `application/problem+json`. Branch on `code`, not `title`.

## MCP

Agents can discover and call this tool through `https://api.miniwebtool.com/mcp` or `https://api.miniwebtool.com/v1/mcp` using MCP `tools/list` and `tools/call`.

## Frequently asked

### What does the Autumnal Equinox Day API do?

Return autumn equinox date, UTC time, seasonal markers, and optional countdown data. Use it to add this utility without rebuilding formulas, validation, examples, and documentation.

### How much does one call cost?

1 credit per successful call. Failed calls do not bill.

### What parameters are required?

Required fields: (none). Full schema at `https://api.miniwebtool.com/v1/openapi.json`.

### Is this deterministic?

Yes. Same input, same output. Tool version: 2026-04-22.