Database Design - Storing Stadium Capacity for Current and Past Events
Three options:

Option One

Create a history table storing all data that can change about the stadium. This could also include its name, though its location is probably set. Lets call this "StadiumHistory". Your Schedule table would then use this as its FK. The StadiumHistory table would have a FK to the Stadium table which uniquely identifies the stadium and its location.

Option Two

Ask yourself "what is my eventual goal?". The answer is probably "to create a system which can be queried really quickly, where calculations including aggregation functions of teams/goals/etc can be performed very quickly to enable me to identify likely results or predict future outcomes based on a variety of factors, or to examine trends etc from the past".

If this is your eventual goal, you may want to keep your 3NF database above plus have it regularly export to another database, one which has dimension normalization - a data warehouse. Your stadium then becomes a Dimension table, probably expanded to "Location" with your results a Fact table. This Dimension table would contain values (which would often repeat) for all of the fields you have specified in Stadium. In fact, you could expand the "Location" table then to include weather information for the day, pitch condition, crowd attendance etc.

This approach has many advantages long term, but requires a new database (development) and an appropriate ETL process to load it. My experience tells me, you will eventually be building this if your database records historical information and you want to begin aggregating information for trend analysis. If you find yourself writing history tables for everything, you will find the interfaces for this difficult to maintain in your current database.

Option Three

Forget about anything that changes. Is it really important that you record the capacity on any day? If not, don't worry about the history of recording stadium capacity. Your database then only contains the most up to date stadium information. 

This is the simplest option - it means you don't have to change your database design.