Saying about a person that he or she has “семь пятниц на неделе” (seven Friday in the week) значит, что человек непостоянен в своем мнении или планах и меняет их по несколько раз! (means that a person is unstable in its opinion, or plans and change them several times).
In old times, Friday was always a very busy day, а потому базарным днём (because it was the market day). Friday was also the day on which various obligations and trade had to be executed. Merchants would take orders and receive deposit, bring goods that had been promised the week before and get paid for their work. On the other hand, clients would come to the market to get their goods and paid the work done. If a merchant or a client broke the deal and didn't turned up, people would say about him that he had but seven Fridays in the week. Later this idiom was applied to people who often change their mind and can't stick to a decision for very long.
(Well, have you decided where to go on vacation? - Not yet - you know Anka, she has seven Fridays in the week. She wanted to go to Thailand, now she wants to visit Chile.)
(Maybe you should determine what you want exactly at first? otherwise you won't be understood, you have seven Fridays in the week.)
Means that a person is unstable in its opinion,or…