How to select the last div based on attribute

I have this html code:

<div id="mydiv">
    [other divs]
    <div data-day="1">content</div>
    [other divs with data-day attribute]
    <div data-day="random">content</div>
    [other divs]
</div>

I wanna select the last element in the mydiv that has data-day attribute. How can I do this?

#mydiv div[data-day]:last-child

I tried this, but it didn't work.