en.javascript.info/2-ui/1-document/07-modifying-document/9-calendar-table/solution.md

We'll create the table as a string: "<table>...</table>", and then assign it to innerHTML.

The algorithm:

1. Create the table header with <th> and weekday names.
2. Create the date object d = new Date(year, month-1). That's the first day of month (taking into account that months in JavaScript start from 0, not 1).
3. First few cells till the first day of the month d.getDay() may be empty. Let's fill them in with <td></td>.
4. Increase the day in d: d.setDate(d.getDate()+1). If d.getMonth() is not yet the next month, then add the new cell <td> to the calendar. If that's a Sunday, then add a newline "</tr><tr>".
5. If the month has finished, but the table row is not yet full, add empty <td> into it, to make it square.