In the past when I've taught Sunday School classes, I've always been amazed how attendance fluctuated very little each week, yet we had different people attending.
We had the "regular" attenders and then we had the "revolving" attenders.
Pastors tell me that the same thing happens for church services. Some people become the "every other week" crowd. Several pastors have shared that often the people revolving come less in the summer. Plus the regulars aren't coming because of vacations or traveling on weekends.
Church attendance usually slumps in the summer. And it's somewhat understandable.
Knowing the way people come to your church should help you communicate to them. Even when they're not coming to the services, most people want to know what's going on at "their" church.
In the summer especially, you should communicate differently knowing that people are not as "regular" as they are during the year.
- E-blasts: Take a summer focus (especially the design) to let people know what's happening at the church during the summer months. Segment your emails based on your list (age, interest, etc) and more will read them. Keep the emails short, be creative with links to your website, and use pictures! Perhaps a subtle online giving link will help remind people of their fiscal responsibility. Or be specific about a financial need and people will give to it.
- Social Media: People keep checking Facebook, Twitter, Instagram, YouTube, etc. even in the summer. In fact, some people check it more! Don't over promote on your pages; instead have fun summer posts like: the Pastor playing golf, the summer picnic photos, or tips to finding a church while on vacation. Occasionally recommend ways to worship at your church even in the summer! Have fun with it and engage. Ask questions, answer comments, and follow the vacation status updates on your member's pages.
- Website: Knowing people are away; make sure there's a "in case you missed this Sunday" link to let them watch the sermon, browse the bulletin, or see the announcements. Maybe have an edited video called "Sunday Service in Half the Time". Keep in mind that people from other churches are traveling to your area and they're "checking out" where to go this Sunday. Give them a nice welcome online and tell them what to expect when they come (what to wear, what they'll hear, etc).
If you keep communicating to your members all summer, they'll be better engaged for ministry in the fall. You may want to tease them a bit about the interesting ministries you're planning in September.