What is the significance of the phrase “二十五，糊窗户”？ The phrase “糊窗户” refers to the practice of pasting paper cutouts (similar to the one seen here) on one's windows. There's a common larger saying, “二十五，糊窗户”, which I assume means that this is usually done on the 25th day of the lunar month. (I'm not sure which one.)

What is the meaning and significance of this phrase? What does it refer to? When is it used?
Traditionally, the Spring Festival starts from 23rd December (lunar month), 23rd December is also called 小年 (little Spring Festival)

There is an old saying...

二十三，祭灶官；二十四，扫房子；二十五，糊窗户 (or 磨豆腐)；二十六，割块肉；二十七，杀只鸡；二十八，贴gaga(嘎嘎音，意为贴春联)；二十九，灌壶酒；年三十，包扁食。

Basically, it describes what people do each day from 小年 to Chinese New Year Eve (三十).