メイドの日 means Maid’s Day, a wordplay on the date May 10. May -> メイ (MEI), 10 -> とお (pronounced as toe), May 10 -> メイとお -> メイド (pronounced as may-doe), and メイド is the transliteration of maid.

On SNS in Japan, there are numerous cases of such wordplay. They are somewhat netizen-made festivals when SNS users use as a reason to post about the topic. For example on today’s Maid’s Day, SNS would be flooded with maid related pictures.

It’s worth mentioning that maid is a popular theme of otaku culture. The maid cafés in Akihabara (or otaku areas in other cities in Japan) are definitely a must go experience.

Learn more about maid café culture on wikipedia.