The data is only available to be used until 23:59 on Sunday 11 May.
"To share our birthday with the nation is truly special,” said Joosub, adding the that the deal comes with no strings attached.
He said that this give-away is the first of many yet to come for Vodacom's over 33 million customers.
"We’re planning a year of celebrations and have new firsts lined up throughout the year."
According to Vodacom's website, 1 GB prepaid data costs R149.
The Independent Communications Authority of South Africa (Icasa) has been trying to drive down the cost of voice and data traffic.
It has already implemented regulations on mobile termination rates, which are the rates operators pay one another for calls to other networks.
Actor Tumisho Masha, together with other Vodacom users, welcomed the free data on Twitter.
?????? yay I got mine! Thanks @Vodacom pic.twitter.com/HOT9damCHT
— Tumisho Masha (@TumishoMasha) May 10, 2014
Wow cool! To celebrate their 20th birthday @Vodacom are giving all their customers 1GB of free data to use this Sunday. (just for the day)
— Aki Anastasiou (@AkiAnastasiou) May 9, 2014
We PROUD to say tomorrow from 00h01 until 23h59 we giving all our customers a FREE 1GB of data to utilise all day long VIVA Vodacom VIVA!
— Okuhle (@just_Okuhle) May 9, 2014
At least one customer wanted the deal to last longer. We will never hear the end of it RT @tshepomagwaba: Hey @vodacom how about we do this data gifting for the whole week just for control lol
— #MamelodiSundownsFC (@N_NMosing) May 11, 2014
Another, termed the offer as "disingenuous".
Why does Vodacom give us data and tell us when to use it. This offer is disingenuous.
— Thapelo (@mokhathi) May 11, 2014
To check your balance on your free 1GB allocation dial *111*600#.How are you using your data? Downloading songs, games, apps, surfing the net, look for a job online. Tell us.
- Fin24