This Saturday is the most popular wedding day of the year, according to data released by the Knot. Almost 30,000 couples are set to marry, with associated guests spending an estimated $1 billion on gifts and attire for the happy events, says the site.

In advance of this very hectic day, the multiplatform wedding-planning brand (part of XO Group Inc.) released its 2018 Guest Study on Tuesday, revealing average attendee expenses per wedding, as well as factors contributing to whether invitees accept or decline, and the thinking behind choosing an appropriate gift.

Per the Knot's membership data, 28,633 weddings are scheduled for Aug. 18 in the U.S.

"A lot of couples picked it for the significance of the date, it's a palindrome month-eight, one, eight, one, eight," says senior editor Ivy Jacobson. She also notes that summer and early fall months are the sweet spot for couples planning a wedding, generally due to weather concerns.

An average of 136 guests per wedding will attend this weekend's celebrations. Those estimated 3.9 million guests will, on average, each spend $261 on the event, including the gift, attire, and accessories, which adds up to an estimated $1 billion weekend-and that's without travel costs. Guests traveling for a wedding spend an average $901 for the event total, including costs for accommodations, travel, gifts, attire and accessories, while members of the wedding party spend $928.

While the average spends above for the wedding day are similar, Jacobson says it's worth taking into account the extra costs involved for those in the wedding party. "You have the additional cost of the bachelor/bachelorette party, engagement party, bridal shower on top, so the cost goes up from there," she says.

Of the 1,337 qualified responses (recruited via Facebook) for the study, 83 percent were a guest invited to attend a wedding celebration and 17 percent were a member of a wedding party, with the demographic breakdown of responders being 89 percent female and 11 percent male.

A national survey by Bankrate.com released in March found similar results, with the total average spend, including travel, on someone else’s happiness to be $728; in the Northeast that number spiked to $1,070.

So while you may be invited to fewer weddings, expenses are rising.

"There can be some sticker shock at first," says Jacobson. "But if you are choosing to spend this money, this person is important to you in some way and people are excited to celebrate." Overall, 70 percent of those surveyed said they enjoyed the last wedding they attended, especially if it was well-organized.

When considering an RSVP, guests say their relationship to the couple is the most important factor regarding attendance (71 percent). Other considerations include date (50 percent), cost of travel (40 percent), whether their children were also invited (42 percent), and if they received a plus one option (34 percent).

As for that all-important gift? Thirty-four percent of the Knot study responders said they purchase a gift off the couple's registry, with 29 percent gifting cash or check, and 10 percent proffering gift cards. On average, members of the wedding party spend $107 on the gift, while wedding guests spend an average of $88. The following five items rank as the most popular gifts for Knot-registered couples on Aug. 18: the KitchenAid stand mixer, Ninja blender, Dyson vacuum, iRobot Roomba, and an air fryer.

After Aug. 18, Saturday, Oct. 6 ranks as the second most popular date in 2018 with 24,359 weddings planned on the Knot. For users of the site, September is the most popular month to wed (165,157 weddings scheduled), ahead of October (152,521) and June (135,140).

Marking 22 years in the business this year, Jacobson says the Knot helps plan 8 out of 10 weddings conducted in the U.S. annually, listing the national average cost per wedding in 2017 at $33,391. If you’re keeping track, that’s $956 million more pumped into the wedding-industrial complex this weekend.