There are a total of 18 10-letter words containing 'day', and the number may vary in different dictionaries. You can specify the dictionary and order.

• daydreamed ₁₈
• daydreamer ₁₇
• dayflowers ₂₀
• daylighted ₁₉
• daysailers ₁₄
• daysailing ₁₇
• daysailors ₁₄
• daysprings ₁₉
• daytrading ₁₇
• dayworkers ₂₀
• doomsdayer ₁₇
• holidayers ₁₆
• holidaying ₁₉
• preholiday ₁₉
• schooldays ₁₉
• unbirthday ₂₀
• yesterdays ₁₅
• daycentres ₁₆

There are 18 10-letter words containing day in the well-known word game dictionary, 12 in the Scrabble US (US/Canada/Thailand - TWL/NWL) dictionary, and 14 in the Scrabble UK (International - SOWPODS/CSW) dictionary, and 17 in the Words with Friends (WWF) dictionary.