A drive-in testing centre for key workers at the Cardiff City Stadium was shut on Bank Holiday Monday, prompting criticism from opposition politicians.
Public Health Wales said it was due to the "low number of key workers anticipated to be working" that day.
But Plaid Cymru leader Adam Price said the decision "beggars belief".
But the Welsh Government said the opposition comments showed a "lack of understanding" of how the system worked.
Meanwhile the Welsh Conservatives said there was no valid reason to have closed the centre.
The Cardiff City Stadium centre is one of four drive-in testing centres planned to provide testing to key-workers with symptoms - it is the only centre operating so far, having opened last week.
Health Minister Vaughan Gething had promised 5,000 tests a day by mid-April - but publicly available data from Public Health Wales shows daily testing figures have not exceeded 939 since last Wednesday.
Figures on Public Health Wales' dashboard suggests the level of daily tests has stayed relatively stable for two weeks, mostly hovering around 800 to 900 since 30 March, and dipping to as low as 507 on 5 April.
As well as key-workers, people who are admitted to hospital with symptoms are tested separately. The Cardiff centre has capacity for 200 tests a day.
In a statement, Public Health Wales said full service at the centre would resume on Tuesday.
"The decision to close the centre was taken due to the low number of key workers anticipated to be working on the Bank Holiday Monday," PHW said.
"In the interests of efficiency it was therefore not deemed necessary to have the test centre operational that day."
Plaid Cymru leader Adam Price said: "During a national public health emergency, when we are already testing far less than many other countries, it beggars belief that the Welsh Government should think it appropriate to close a testing station for the Bank Holiday.
"We were promised that we would be at 5,000 tests a day by mid-April. It is not surprising with this kind of approach that we have made absolutely no progress towards that figure so far."
Welsh Conservative health spokeswoman Angela Burns said the decision was "incredible".
"While I appreciate that resources - especially staff - are stretched now, keeping the doors of a test centre shut on the assumption that few key workers would turn up to be tested is, I think, based on a false premise."
Tests not carried out on Monday "will just add to the burden of tests needing to be taken and then analysed", she said.
A Welsh Government spokesman said: "This shows a complete lack of understanding of how the system workers. Key workers can't just turn up to this centre, they have to be booked in.
"Because of the small number booked in for today, they were all tested yesterday instead.
"Testing happens in a variety of locations and we now have a capacity to test up to 1,300 a day."