$day = date('w');
$week_start = date('m-d-Y', strtotime('-'.$day.' days'));
$week_end = date('m-d-Y', strtotime('+'.(6-$day).' days'));

{% set day = now|date('w') %}
{% set weekStart = now|date_modify("-#{day} days")|date('m-d-Y') %}
{% set weekEnd = now|date_modify("+#{6-day} days")|date('m-d-Y') %}

{{ now | date_modify('monday this week') | date('m-d-Y') }}

{{ now | date_modify('sunday this week') | date('m-d-Y') }}

{{ now | date_modify('last day of this month') | date('m-d-Y') }}