Lots of companies start to increase their rate of hiring in January and February, when managers want to get teams and plans in place before the end of the financial year and for the calendar year ahead. Hiring tends to remain brisk through Spring.
Job availability tends to drop in Summer, when fewer people are hiring, and it usually picks up again in the Autumn before dropping again in the holiday season.
These rules are true for lots of companies, but there are always exceptions. Lots of companies work on different seasonal schedules, while others keep hiring all year round. So while there are noticeable peaks and troughs in hiring, these rules aren’t steadfast, and jobs will always be available.