"The ELN will cease military activities from midnight on May 25 to midnight on the (May) 29th to bring about favorable conditions to allow Colombian society to express itself," the group wrote on Twitter.
They added that "after announcing an end to military activities for the elections...we hope to see a similar stance from the various political and military forces of the regime."
The armed group announced a similar ceasefire during the last legislative elections on March 11.
The government of Santos has been involved in a peace process with the ELN, currently in its fifth round of negotiations in Havana, Cuba.
President Santos on Thursday said that he was confident that negotiations with the ELN would "end well."
"I have faith...that this process will also end well. It will be difficult but there is no other choice. I believe the ELN also understands that there is no other choice...we will try to reach a ceasefire as soon as possible," he said.
Negotiations previously took place in Quito, Ecuador, but President Lenin Moreno pulled out of hosting the peace talks after dissident members of the FARC, an extinct Colombian guerrilla group, killed three Ecuadorian journalists earlier this year.