Google Calendar vs Non-Working Days

The Google Calendar API reads and writes events, calendars, free/busy data, and reminders for any Google account that's granted OAuth consent, with push-notification channels available so an app can watch a calendar for changes instead of polling it. It's the same REST API that powers Calendar add-ons and third-party scheduling tools like Calendly. Usage is free under Google's shared per-minute and per-project quotas (10,000 requests/minute/project as of the May 2026 quota update); there's no separate paid tier for the API itself, only future overage billing above a 1,000,000-requests-a-day threshold.

isdayoff.ru answers one narrow question extremely cheaply: for a date or date range, is it a working day, a weekend, or an official non-working holiday. Responses are bare digit strings (0/1 per day, or 2 for a shortened day) rather than JSON, which keeps payloads tiny for the batch/range endpoint. It documents Russia, Belarus, Ukraine, Kazakhstan, and the US as supported country codes, but live probes this run found only Russia (the default), Belarus, and Kazakhstan return genuinely varying data — the Ukraine and US codes are accepted (HTTP 200) but answered every single day of several different months with a flat, undifferentiated "0". It's free, keyless, and has no published rate limit, built for production-calendar bookkeeping rather than general holiday metadata.