The July 2017 New Moon in Leo occurs at 5:46 AM EST on Sunday July 23rd, and your New Moon ritual can be done anytime after 5:46 AM on the 23rd and all day Monday the 24th.
On Sunday the 25th the Moon goes void-of-course at 2:44 PM then enters Leo at 6:06 PM, but your New Moon ritual should be performed before then, ideally. A lot of other planets are in Cancer for this Moon as well, so expect a big wave a Cancer energy coming.
New Moons are used for setting intentions with a manifestation point at the next Full Moon which happens in about two weeks, August 7th for this New Moon. Also, and way more importantly, there is a second, larger manifestation at the Full Moon that happens around six months later in the same sign. For this New Moon its six month Full Moon will happen on January 31st 2018.
My guide, Witch’s Guide to New Moon Magick, can be found on the site here; this post is a companion to that guide. If you’re new to Moon magick that’s a great place to begin.