Hi and welcome
reut.
There are no "exact dates" in gardening really. There are times of year that are better for planting new plants than others. Usually the best time of year for planting are springtime, after the last frost or in the fall when the plants are about to go dormant.
For plants that will be growing in the shade, such as violets it's probably fine to plant them now as long as you can be sure to keep them watered. But with the most intense sun of the year coming up, and the two hot months - mid June to mid August upon us this is often a tough time to get plants to root and settle in. Cooler weather and cool soil gives them a chance to develop roots and get established whereas in hot summer weather they want to put on top growth at the expense of root development.
To be on the safe side, I'd wait to plant until September but if you are anxious to get started, shade your new plantings and keep them well watered until they begin new growth, then keep them as cool as you can and gradually acclimate them to more light. Water well in the mornings to guard against fungal disease and hydrate the plants thoroughly for the hot days to come.
