The version I originally started with has first day of week as Monday, but I need it to be Sunday:

<script>
    import { beforeUpdate } from 'svelte';

    import {
        startOfMonth,
        addMonths,
        format,
        subMonths,
        addDays,
        startOfWeek,
        sub,
        add,
        eachDayOfInterval,
        getDay,
        isToday
    } from 'date-fns';

    let firstDayOfWeek;
    let currentMonth;
    let nextMonth;
    let previousMonth;
    let daysOfCurrentMonth;
    let weekdayOffset;
    let weekNames;

    beforeUpdate(() => {
        firstDayOfWeek = startOfWeek(new Date(), {
            locale: navigator?.language.split('-').pop().toLowerCase() || 'us',
            weekStartsOn: 0
        });
        currentMonth = startOfMonth(new Date());
        nextMonth = startOfMonth(addMonths(new Date(currentMonth), 1));
        previousMonth = startOfMonth(subMonths(new Date(currentMonth), 1));
        daysOfCurrentMonth = eachDayOfInterval({
            start: currentMonth,
            end: sub(nextMonth, { days: 1 })
        });
        weekdayOffset = (getDay(currentMonth) + 7) % 7 || 7;
        weekNames = [...Array(7)].map((_, index) => format(addDays(firstDayOfWeek, index), 'EEEEEE'));
    });
</script>

{#if weekNames}
    <div class="week-days">
        {#each weekNames as weekName}
            <p>{weekName}</p>
        {/each}
    </div>
{/if}

{#if daysOfCurrentMonth}
    <div class="days">
        {#each daysOfCurrentMonth as day, index}
            <p
                class="day"
                style={`
                grid-column: column(${index});
                grid-column-start: ${index === 0 ? weekdayOffset : 0};
                color: ${isToday(day) ? 'red' : 'white'};
            `}
            >
                {format(day, 'dd')}
            </p>
        {/each}
    </div>
{/if}

<style>
    .days,
    .week-days {
        display: grid;
        grid-template-columns: repeat(7, 50px);
        grid-column: 7;
    }
</style>

I believe the problem resides with this line:

weekdayOffset = (getDay(currentMonth) + 7) % 7 || 7;

It has Feb 1, 2022 starting on Monday when it should be Tuesday.

Here is a fiddle: https://codesandbox.io/s/naughty-surf-bovu8?file=/Button.svelte