From the Electoral Commission:
On 20 February, the Prime Minister announced that the UK Government’s proposed date for the referendum on the UK’s membership of the European Union is 23 June 2016. Regulations will now need to be debated and approved by the UK Parliament before the date of the poll is confirmed.
The proposed referendum period is:
Start of referendum period: 15 April*
Polling day: 23 June
The proposed designation period is:
Application period starts: 4 March
Deadline for applications: 31 March
Designation decision: 14 April**…
There are restrictions on the material that the UK and devolved Government ministers, government departments, local authorities and certain bodies
receiving public funds can publish about the referendum from 27 May to 23 June 2016, polling day. The restrictions apply to any material, made available to the public, in any form and by whatever means which provides general information about the referendum as well as the issues and arguments for or against the UK remaining or leaving the EU. The restrictions also apply to material designed to encourage people to vote.
* This is therefore the date on which extra restrictions on campaign activities kick in.
** This is the date on which the appointment of the official Remain / Leave campaigns is announced.
UPDATE: See here for the full European referendum timetable.