Bartholomew Dias and his crew landed with his men on 3rd of February 1488, but Mossel Bay's History can be traced back, with the help of Archaeological deposits, to more than 164 000 years. They have since built a modern-day Dias Museum Complex, on the site Dias landed. Here they found a spring from which to replenish their water supplies. Dias had been appointed to search for a trading route to India, by King John II of Portugal. Without realizing that he had actually rounded the Cape of Good Hope before landing at Mossel Bay. He named the Bay- Anga dos Vaqueiros, which means the Bay of Cowherds. Dias is also credited with having given the Cape the name, Cabo da Boa Esperanca. This means; the Cape of Good Hope. Dias was however, chased off in a hail of stones by the local peoples!

By the time the Portuguese explorer Vasco da Gama reached the area on the 3rd of February 1497, the Bay had been recorded as Aguada da Sao Bras. The watering place of St. Blaize- a Saint whose Feast Day is celebrated on the 3rd of February!

Da Gama bartered successfully for cattle with the Khoi people, in what is regarded as the first transaction between the Europeans and the indigenous people of South Africa.

In 1501, another Portuguese navigator, Pedro d'Ataide, sought shelter in Mossel Bay after losing most of his fleet in a storm. He left an account of the disaster hidden in an old shoe, which he suspended from a milk wood tree near the spring from which Dias had drawn his water. The report was found by the explore to whom it was addressed, Joao da Nova. The tree tree has served as kind of a post box for decades afterwards. Recently a boot-shaped post box has been erected under the now famous Post Office Tree and letters posted there are decorated with a commemorative stamp! This has ensured that the tree has remained one of the towns biggest tourist attractions!

The first post box Image source

Joao da Nova erected a small shrine near the Post Office Tree and although no traces remain, it is considered the first place of Christian worship in South Africa.

The origin of the name Mossel Bay, which means bay of mussels, has to do with the various Dutch shipping merchants. In the late 16th and early 17th Centuries there is an accountof the explorer 

Geolocation
22° 2' 27.6", -34° 8' 60"
References

New Dictionary of South African Place Names by Peter E. Raper