The rooftop observatory of Shibuya Sky (called Sky Stage) limits the number of people who can enter every 20 minutes between 9am to 10pm and you can only enter at the time shown on the ticket (Once you enter, you can stay as long as you want). I recommend that you buy tickets beforehand, but you can also do so on the 14th floor when you visit, as of December 2019. On a sunny day, you can enjoy a panoramic view of the Tokyo metropolitan area at a height of 230 meters above Shibuya, so remember to check out the weather forecast in the morning!
For your information, Tokyo on average has 40 clear days (approx. 11%) and 220 sunny days with scattered clouds (60%). If the forecast says it will be a sunny day, buy tickets online and head to Shibuya. Admission is 2,000JPY for an adult, but 1,800 JPY if you purchase online.