The Colombian Congress has ratified the peace agreement between the government of Juan Manuel Santos and the Revolutionary Armed Forces of Colombia, but demobilization of rebels has yet to begin over an ongoing disagreement over what the accord calls “D-Day,” which is when the combatants are to formally begin gathering in demobilization zones.

Colombian President Santos remarked that Thursday marked “the first day of peace” and his government expected demobilization to begin soon. The accord specifies that five days after “D-Day” the rebel combatants are expected to begin reporting to the “concentration zones” where the demobilization process will take place.

However, FARC leader and peace negotiator Pastor Alape said “D-Day” has not yet been determined and rebels won't moving to the concentration zones until security can be guaranteed. Colombia has been recently experiencing a spike in killings of leftist activists and the FARC leadership is concerned that their members will be assassinated.

The FARC is also concerned that they will face arrest for their rebellion as an amnesty law has not yet been passed by Congress. The Congress must also approve a series of other laws that would allow for the concrete implementation of the historic peace agreement.

The rebels want these laws to be approved quickly through a “fast track” mechanism before they begin moving to the concentration zones.

Colombia's Constitutional Court is expected to rule Friday regarding the legality of the fast track process. This process, which would allowed for fewer rounds of voting, was originally tied to a successful plebiscite result but Colombian narrowly rejected the peace agreement in a vote. The updated peace agreement was not put to vote and was approved directly by Congress.

If the Constitutional Court says the fast track is not valid, then the series of laws that will allow for the implementation must go through the regular legislative process, which could take many months, further delaying the demobilization of combatants.

The urgency for demobilization stems from the fact that the bilateral cease-fire is fragile, as two FARC rebels were recently gunned-down by state security forces.

According to a report by the Washington Office on Latin America, or WOLA, there is also concern that too much idle time may lead to desertions by FARC rebels. The FARC also controls or holds influence over many areas, and the cease-fire has meant a reduced presence by rebels creating a vacuum that may be filled by other armed actors.