Learn Turkish: Tatil mi planlayacaktınız? - Were you going to plan a holiday?

It combines the future suffix with the past tense. Start with the verb root planla- (to plan) and add the buffer -y-. Then add the future suffix -acak (will), the past suffix -tı (was/were), and the plural/formal "you" ending -nız. Together, the combination -acaktı creates the "was/were going to do" meaning.

Why is the question particle mi placed after tatil instead of at the end of the sentence?

In Turkish yes/no questions, the word immediately before the question particle gets the emphasis. By asking Tatil mi planlayacaktınız?, the speaker is specifically asking "Were you going to plan a holiday?" (as opposed to a party, a meeting, etc.). If they wanted to question the action itself, the particle would go with the verb: Tatil planlayacak mıydınız?.

Why doesn't the k in the future suffix -acak change to ğ here?

The letter k only softens to ğ when it is followed by a vowel, such as in planlayacağım (I will plan). In this sentence, the suffix right after -acak is the past tense -tı, which starts with a consonant. Because it's followed by a consonant, the k stays hard.

Why is it just tatil and not tatili?