This is a question you should be asking yourself before sending funds over the Ethereum network. While some days are better for lower fees than others, aiming to use alternative blockchains such as Polygon (MATIC) or BSC are perfectly good options for crypto transactions, which will save tons in terms of transaction fees.
However, if you still want to use Ethereum for sending funds, it is best to perform transactions over the weekend or early in the morning between 1 and 3 am UTC.
If you want to pay the lowest Ethereum fees, you should perform transactions early in the morning, between 1 and 3 am UTC or late at night, between 9 and 11 pm UTC.
For our german reader: its 2:00 am to 4:00 in the night Berlin Time. It should be between 22:00 and 23:00 o clock