MCP connector for Claude

Every federal holiday, and the business days between them.

Holiday Calendar (example) by Modern Mustard Seed answers the date questions a small business asks every week: when is the next federal holiday, is this a business day, what date is 10 business days from the invoice, how many working days are left this quarter. It computes the eleven U.S. federal holidays from the statute that sets them, 5 U.S.C. 6103, with the weekend shifts federal offices observe. No model guesses anything and nothing leaves the server.

Connector URL

https://starter.modernmustardseed.com/holidays

Streamable HTTP. No sign-in. Every tool is read-only and makes no network calls.

Connect it

In Claude, open Customize, then Connectors.
Choose Add custom connector, paste https://starter.modernmustardseed.com/holidays, and save. No account or key is needed.
In a chat, turn the connector on from the + menu and ask a date question. Claude calls the tools when the question needs them.

In Claude Code: claude mcp add --transport http holidays https://starter.modernmustardseed.com/holidays

Try these

“Which federal holidays fall on a weekend in 2027, and which days are they observed instead?”
“An invoice dated 2026-11-20 is due in 10 business days. What date is that, and which holidays did you skip?”
“How many business days are there between 2026-12-01 and 2027-01-15?”

The tools

list_federal_holidays

List federal holidays

List the eleven U.S. federal holidays for one calendar year (2021 through 2100): each holiday's name, its actual date and weekday, the date it is observed when it falls on a weekend (Saturday moves to the Friday before, Sunday to the Monday after), and the rule that sets it, from 5 U.S.C. 6103. Use it to answer which days federal offices close in a year.

Inputs: year

next_federal_holidays

Find the next federal holidays

Find the next one to eleven U.S. federal holidays observed on or after a date, with each one's observed date and how many calendar days away it is. Without from_date it starts from today's date in America/New_York and says which date it used.

Inputs: from_date, count

check_business_day

Check a business day

Check whether a date is a U.S. business day (a weekday that is not an observed federal holiday). Returns the weekday, the holiday observed that day if any, and the business days just before and just after it.

Inputs: date

add_business_days

Add business days

Move a number of U.S. business days forward or backward from a date, skipping weekends and observed federal holidays, for due dates, delivery windows and notice periods. The start date is not counted: 1 business day after a Friday is the next Monday, or Tuesday when that Monday is a holiday. Returns the resulting date and every holiday it skipped. Up to 1,000 business days either way.

Inputs: start_date, business_days

count_business_days

Count business days

Count the U.S. business days between two dates, both dates included, leaving out weekends and observed federal holidays. Returns the business day count, the weekend days, and each holiday that fell in the span. Spans up to 3,660 calendar days.

Inputs: start_date, end_date

The rules it follows

The holidays are the eleven in 5 U.S.C. 6103(a): New Year's Day, the Birthday of Martin Luther King, Jr., Washington's Birthday, Memorial Day, Juneteenth National Independence Day, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving Day and Christmas Day.
A holiday on a Saturday is observed the Friday before; on a Sunday, the Monday after. New Year's Day on a Saturday is therefore observed on December 31 of the year before.
A business day is a Monday through Friday that is not an observed federal holiday.
Adding business days never counts the start date. Counting business days includes both ends.
States, banks, schools and private employers can observe other days. The Federal Reserve stays open on the Friday before a Saturday holiday, and Inauguration Day is a holiday only for federal workers around Washington, D.C. Every answer says so.

Limits

Dates from 2021 through 2100. Juneteenth became a federal holiday in 2021, so earlier years follow a different list.
add_business_days moves up to 1,000 business days either way. count_business_days spans up to 3,660 calendar days.
Without a start date, next_federal_holidays uses today's date in America/New_York and says so in its answer.
Each tool answers at most 300 calls a minute on each server instance, and the service at most 3,000 a minute in total. Over a limit, the tool says so and gives the time to try again.

Troubleshooting

“must be a date written YYYY-MM-DD”: Dates are calendar dates in ISO form, such as 2026-11-26. Ask Claude to use that form, or give the date in words and Claude converts it.
A holiday seems to be missing: Check the observed date, not the actual one. Independence Day 2026 falls on a Saturday, so it is observed on Friday, July 3.
My bank or state closes on a different day: This connector covers federal holidays only. Banks, states and employers publish their own calendars.
Claude says it couldn't reach the server: Remove the connector and add it again with the exact URL above, no trailing slash. If it still fails, write to support with the time it happened.

For reviewers

No account, key or setup is needed. Add the connector URL above, then:

list_federal_holidays with {"year": 2027} returns eleven holidays; Juneteenth and Independence Day show Friday and Monday observed dates.
add_business_days with {"start_date": "2026-11-20", "business_days": 10} returns 2026-12-07 and lists Thanksgiving as skipped.
count_business_days with {"start_date": "2026-12-01", "end_date": "2027-01-15"} returns 32 business days.
check_business_day with {"date": "2021-12-31"} returns New Year's Day 2022, observed.
next_federal_holidays with {"from_date": "2026-10-11", "count": 3} returns Columbus Day, Veterans Day and Thanksgiving.
Errors: {"date": "2026-02-30"} and {"year": 2019} each return a specific message.

Support and security

Questions, problems and security reports go to sarah@modernmustardseed.com. A machine-readable contact is at /.well-known/security.txt. How data is handled is in the privacy policy, and the rules of use are in the terms.