Every year, the UK tax year runs from the 6th of April, until the 5th of April. This means you have until April 5th to max out your ISA allowance across all of your ISAs you may have open.
Your ISA allowance includes any money you contribute across all of your ISAs in the same tax year. For example, you could add £16,000 into a Stocks & Shares ISA (the type of ISA that CIRCA5000 offers), meaning you would only have £4,000 left to add to a different type of ISA, for example a Cash ISA.
If you have the means to maximise your ISA allowance, now is the time to do it!