There are tennis tournaments almost all year round but the 4 main ones that players try and aim for are:
1. Australian Open played in January each year in Melbourne
2. French Open played in May/June each year at Roland Garros in Paris
3. Wimbledon played in June/July each year at Wimbledon in London
4. US Open is played in August/September each year in New York.
There are also team tournaments such as the Davis Cup, and ATP Cup played at the end of each season with the top 8 players and the Olympics which are considered important tournaments too.