勤労感謝の日は元々収穫祭である新嘗祭だったらしいですよ！
ごはんを神様に感謝！　【アンケート　主食としてよく食べる（食べたい）穀物は？】

Apparently, Labor Thanksgiving Day was originally a harvest festival, "Niinamesai":http://eos.kokugakuin.ac.jp/modules/xwords/entry.php?entryID=890, the New Crops Festival! Thank the gods with rice!

[Survey: Which grain do you often eat (or want to eat) as a staple food?]
• Wheat
• Soybeans
• Potatoes
• Rice