If it is just for the summer make sure you tell them that. They may be more than willing to take the Mondays off as their vacation. Make it sound appealing to them. If you take Mondays off will you still take a full week or two off? If not them tell them that due to your lighter load on Mondays and Fridays you have decided that in place of 1 or 2 weeks of solid vacation you will take every Monday off until September. This means they don't have to coordinate the weeks you take off with their holidays or find alternate care. Spin it so they win too.