Describe the bug

When a Calendar is created with a locale that has Mondays as firstDay, such as en-GB, it skips the first day of the month when it is a Monday (when viewing the previous month). You only see this when fixedWeek: true is set as true, otherwise the last row is not rendered, of course.

It is difficult to explain well but I have a reproduction: stackblitz.com/edit/github-qbikvw. If you view June 2024, for example, you will see that the bottom row skips 1 July and shows 2-8 July (even though it should only go until 7 July). Or you can go to August 2025 where you see that the bottom row again skips 1 September.

Edit: this is still a problem with locales that have Sundays as firstDay (see August 2024 with the locale set to 'en-US')

Reproduction

https://stackblitz.com/edit/github-qbikvw?file=src/lib/components/Calendar.svelte

Logs

No response

System Info

System:
    OS: macOS 14.4.1
    CPU: (10) arm64 Apple M2 Pro
    Memory: 120.80 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 21.0.0 - ~/.nvm/versions/node/v21.0.0/bin/node
    npm: 10.2.0 - ~/.nvm/versions/node/v21.0.0/bin/npm
    bun: 1.1.7 - /opt/homebrew/bin/bun
  Browsers:
    Safari: 17.4.1
  npmPackages:
    @melt-ui/svelte: ^0.80.0 => 0.80.0
    @sveltejs/kit: ^2.5.2 => 2.5.2
    svelte: ^4.2.12 => 4.2.12

Severity

blocking an upgrade